diff --git a/frontend/app/dataset/[id]/page.tsx b/frontend/app/dataset/[id]/page.tsx
index da7164f..d9ee1b6 100644
--- a/frontend/app/dataset/[id]/page.tsx
+++ b/frontend/app/dataset/[id]/page.tsx
@@ -9,6 +9,8 @@ import { api } from "@/convex/_generated/api";
 import type { Id } from "@/convex/_generated/dataModel";
 import { DatasetTable } from "@/components/table";
 import { useSelection } from "@/components/table/use-selection";
+import { SideSheet, CellDetail } from "@/components/SideSheet";
+import type { DatasetColumn } from "@/components/table/types";
 import { useTheme } from "@/components/ThemeToggle";
 import { StatusBadge } from "@/components/dataset/StatusBadge";
 import { downloadCSV, downloadXLSX } from "@/lib/export";
@@ -35,6 +37,11 @@ export default function DatasetPage() {
   const [settingsOpen, setSettingsOpen] = useState(false);
   const [confirmPopulate, setConfirmPopulate] = useState(false);
   const [savingRefreshCadence, setSavingRefreshCadence] = useState(false);
+  const [cellDetail, setCellDetail] = useState<{
+    column: DatasetColumn;
+    value: unknown;
+    sources?: string[];
+  } | null>(null);
 
   const datasetId = params.id as Id<"datasets">;
   const dataset = useQuery(
@@ -83,6 +90,14 @@ export default function DatasetPage() {
     }
   }, [dataset, populating, getToken]);
 
+  const handleCellExpand = useCallback((columnName: string, value: unknown, rowId: string) => {
+    if (!dataset || !rows) return;
+    const col = dataset.columns.find((c) => c.name === columnName);
+    if (!col) return;
+    const row = rows.find((r) => r._id === rowId);
+    setCellDetail({ column: col, value, sources: row?.sources });
+  }, [dataset, rows]);
+
   const openedFired = useRef<string | null>(null);
   const autoPopulateFired = useRef<string | null>(null);
   useEffect(() => {
@@ -385,8 +400,19 @@ export default function DatasetPage() {
         rows={rows}
         datasetId={datasetId}
         selection={selection}
+        onCellExpand={handleCellExpand}
       />
 
+      <SideSheet open={cellDetail !== null} onClose={() => setCellDetail(null)}>
+        {cellDetail && (
+          <CellDetail
+            column={cellDetail.column}
+            value={cellDetail.value}
+            sources={cellDetail.sources}
+          />
+        )}
+      </SideSheet>
+
       {confirmPopulate && (
         <ConfirmPopulateModal
           rowCount={rows.length}
diff --git a/frontend/components/SideSheet.tsx b/frontend/components/SideSheet.tsx
new file mode 100644
index 0000000..215d477
--- /dev/null
+++ b/frontend/components/SideSheet.tsx
@@ -0,0 +1,228 @@
+"use client";
+
+import { useEffect, useRef, useState, useCallback } from "react";
+import type { DatasetColumn } from "@/components/table/types";
+
+function IconX() {
+  return (
+    <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
+      <path d="M18 6 6 18"/><path d="m6 6 12 12"/>
+    </svg>
+  );
+}
+function IconCopy() {
+  return (
+    <svg width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
+      <rect width="14" height="14" x="8" y="8" rx="2" ry="2"/><path d="M4 16c-1.1 0-2-.9-2-2V4c0-1.1.9-2 2-2h10c1.1 0 2 .9 2 2"/>
+    </svg>
+  );
+}
+function IconCheck() {
+  return (
+    <svg width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
+      <path d="M20 6 9 17l-5-5"/>
+    </svg>
+  );
+}
+function IconExternalLink() {
+  return (
+    <svg width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
+      <path d="M15 3h6v6"/><path d="M10 14 21 3"/><path d="M18 13v6a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h6"/>
+    </svg>
+  );
+}
+
+/* ------------------------------------------------------------------ */
+/*  Shell                                                               */
+/* ------------------------------------------------------------------ */
+
+interface SideSheetProps {
+  open: boolean;
+  onClose: () => void;
+  children: React.ReactNode;
+}
+
+export function SideSheet({ open, onClose, children }: SideSheetProps) {
+  const panelRef = useRef<HTMLDivElement>(null);
+
+  // Lock body scroll while open.
+  useEffect(() => {
+    if (!open) return;
+    const prev = document.body.style.overflow;
+    document.body.style.overflow = "hidden";
+    return () => { document.body.style.overflow = prev; };
+  }, [open]);
+
+  // Keyboard: Escape closes, Tab stays trapped inside.
+  useEffect(() => {
+    if (!open || !panelRef.current) return;
+    const panel = panelRef.current;
+
+    const focusable = panel.querySelectorAll<HTMLElement>(
+      'button, [href], input, select, textarea, [tabindex]:not([tabindex="-1"])',
+    );
+    focusable[0]?.focus();
+
+    function onKey(e: KeyboardEvent) {
+      if (e.key === "Escape") { onClose(); return; }
+      if (e.key !== "Tab" || focusable.length === 0) return;
+      const first = focusable[0];
+      const last = focusable[focusable.length - 1];
+      if (e.shiftKey && document.activeElement === first) {
+        e.preventDefault(); last.focus();
+      } else if (!e.shiftKey && document.activeElement === last) {
+        e.preventDefault(); first.focus();
+      }
+    }
+
+    panel.addEventListener("keydown", onKey);
+    return () => panel.removeEventListener("keydown", onKey);
+  }, [open, onClose]);
+
+  if (!open) return null;
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center p-4" role="dialog" aria-modal="true">
+      {/* Backdrop */}
+      <div
+        className="absolute inset-0 bg-foreground/20 backdrop-blur-sm"
+        onClick={onClose}
+      />
+      {/* Modal */}
+      <div
+        ref={panelRef}
+        aria-labelledby="cell-detail-title"
+        className="relative w-full max-w-lg max-h-[80vh] bg-surface border border-border rounded-xl shadow-2xl flex flex-col"
+      >
+        <div className="flex items-center justify-between px-5 py-4 border-b border-border shrink-0">
+          <h2 id="cell-detail-title" className="text-sm font-semibold text-foreground">Cell Detail</h2>
+          <button
+            onClick={onClose}
+            className="inline-flex items-center justify-center h-7 w-7 text-muted hover:text-foreground transition-colors rounded"
+            aria-label="Close"
+          >
+            <IconX />
+          </button>
+        </div>
+        <div className="flex-1 overflow-y-auto p-5">{children}</div>
+      </div>
+    </div>
+  );
+}
+
+/* ------------------------------------------------------------------ */
+/*  Content                                                             */
+/* ------------------------------------------------------------------ */
+
+interface CellDetailProps {
+  column: DatasetColumn;
+  value: unknown;
+  /** Row-level sources stored by the populate agent. */
+  sources?: string[];
+}
+
+function isValidHttpUrl(src: string): boolean {
+  try {
+    const { protocol } = new URL(src);
+    return protocol === "http:" || protocol === "https:";
+  } catch {
+    return false;
+  }
+}
+
+export function CellDetail({ column, value, sources }: CellDetailProps) {
+  const [copied, setCopied] = useState(false);
+  const copyTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+  const displayValue = value == null || value === "" ? "—" : String(value);
+
+  // Clear any pending timer when the component unmounts to avoid calling
+  // setCopied on an already-gone component.
+  useEffect(() => {
+    return () => {
+      if (copyTimerRef.current != null) clearTimeout(copyTimerRef.current);
+    };
+  }, []);
+
+  const handleCopy = useCallback(async () => {
+    try {
+      await navigator.clipboard.writeText(value == null ? "" : String(value));
+      setCopied(true);
+      if (copyTimerRef.current != null) clearTimeout(copyTimerRef.current);
+      copyTimerRef.current = setTimeout(() => setCopied(false), 2000);
+    } catch {
+      // Clipboard API unavailable (e.g. non-HTTPS dev); silently ignore.
+    }
+  }, [value]);
+
+  return (
+    <div className="space-y-6">
+      {/* Column name + description */}
+      <div>
+        <p className="text-sm font-semibold text-foreground">{column.name}</p>
+        {column.description && (
+          <p className="text-xs text-muted mt-0.5">{column.description}</p>
+        )}
+      </div>
+
+      {/* Value */}
+      <div>
+        <div className="flex items-center justify-between mb-1.5">
+          <p className="text-[11px] font-medium text-muted uppercase tracking-wide">
+            Value <span className="normal-case">({column.type})</span>
+          </p>
+          <button
+            type="button"
+            onClick={() => { void handleCopy(); }}
+            className="inline-flex items-center gap-1.5 text-[11px] font-medium text-muted hover:text-foreground transition-colors"
+            aria-label="Copy value"
+          >
+            {copied
+              ? <><span className="text-green-500"><IconCheck /></span><span className="text-green-500">Copied</span></>
+              : <><IconCopy /><span>Copy</span></>
+            }
+          </button>
+        </div>
+        <div
+          className="rounded-lg border border-border bg-background px-4 py-3"
+          data-ph-mask-text="true"
+        >
+          <p className="text-sm text-foreground break-all whitespace-pre-wrap">
+            {displayValue}
+          </p>
+        </div>
+      </div>
+
+      {/* Sources */}
+      {sources && sources.length > 0 && (
+        <div>
+          <p className="text-[11px] font-medium text-muted uppercase tracking-wide mb-1.5">
+            Sources
+          </p>
+          <ul className="space-y-1.5">
+            {sources.map((src, i) => (
+              <li key={src || i}>
+                {isValidHttpUrl(src) ? (
+                  <a
+                    href={src}
+                    target="_blank"
+                    rel="noopener noreferrer"
+                    className="flex items-start gap-1.5 text-xs text-link hover:underline break-all"
+                    data-ph-mask-text="true"
+                  >
+                    <span className="mt-0.5 shrink-0"><IconExternalLink /></span>
+                    {src}
+                  </a>
+                ) : (
+                  <span className="flex items-start gap-1.5 text-xs text-muted break-all" data-ph-mask-text="true">
+                    <span className="mt-0.5 shrink-0"><IconExternalLink /></span>
+                    {src}
+                  </span>
+                )}
+              </li>
+            ))}
+          </ul>
+        </div>
+      )}
+    </div>
+  );
+}
diff --git a/frontend/components/table/DataRow.tsx b/frontend/components/table/DataRow.tsx
index 1fe0974..2d54661 100644
--- a/frontend/components/table/DataRow.tsx
+++ b/frontend/components/table/DataRow.tsx
@@ -7,12 +7,21 @@ import type { DatasetRow, DatasetColumn } from "./types";
 import { CellValue } from "./CellValue";
 import { floorWidth } from "./utils";
 
+function IconMaximize2() {
+  return (
+    <svg width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
+      <polyline points="15 3 21 3 21 9"/><polyline points="9 21 3 21 3 15"/><line x1="21" y1="3" x2="14" y2="10"/><line x1="3" y1="21" x2="10" y2="14"/>
+    </svg>
+  );
+}
+
 export interface DataRowData {
   rows: Row<DatasetRow>[];
   columns: DatasetColumn[];
   columnWidths: number[];
   isSelected: (id: string) => boolean;
   toggleRow: (id: string, shiftKey: boolean) => void;
+  onCellExpand: (columnName: string, value: unknown, rowId: string) => void;
   isBuilding: boolean;
   pendingRowIds: Set<string>;
   flashingCells: Set<string>;
@@ -27,7 +36,7 @@ function DataRowImpl({
   index: number;
   style: CSSProperties;
 }) {
-  const { rows, columns, columnWidths, isSelected, toggleRow, isBuilding, pendingRowIds, flashingCells } = data;
+  const { rows, columns, columnWidths, isSelected, toggleRow, onCellExpand, isBuilding, pendingRowIds, flashingCells } = data;
   const row = rows[index];
 
   if (!row) {
@@ -112,7 +121,7 @@ function DataRowImpl({
           <div
             key={col.name}
             data-ph-mask-text="true"
-            className={`relative shrink-0 overflow-hidden text-ellipsis whitespace-nowrap border-r border-border/30 last:border-r-0 ${
+            className={`group relative shrink-0 overflow-hidden text-ellipsis whitespace-nowrap border-r border-border/30 last:border-r-0 ${
               cellIdx === 0
                 ? "font-medium text-foreground"
                 : "text-foreground/70"
@@ -123,6 +132,17 @@ function DataRowImpl({
             }}
           >
             <CellValue value={value} type={col.type} />
+            <button
+              type="button"
+              onClick={(e) => {
+                e.stopPropagation();
+                onCellExpand(col.name, value, row.original._id);
+              }}
+              className="absolute right-1 top-1/2 -translate-y-1/2 opacity-0 group-hover:opacity-100 focus-visible:opacity-100 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-foreground p-0.5 rounded bg-foreground/5 hover:bg-foreground/10 text-muted hover:text-foreground transition-all"
+              aria-label={`Expand ${col.name}`}
+            >
+              <IconMaximize2 />
+            </button>
             {isPending && <div className="shimmer-overlay absolute inset-0" />}
           </div>
         );
diff --git a/frontend/components/table/DatasetTable.tsx b/frontend/components/table/DatasetTable.tsx
index f921875..e74c2bb 100644
--- a/frontend/components/table/DatasetTable.tsx
+++ b/frontend/components/table/DatasetTable.tsx
@@ -85,11 +85,13 @@ export function DatasetTable({
   rows,
   datasetId,
   selection,
+  onCellExpand,
 }: {
   dataset: DatasetMeta;
   rows: DatasetRow[];
   datasetId: string;
   selection: Selection;
+  onCellExpand: (columnName: string, value: unknown, rowId: string) => void;
 }) {
   const tableContainerRef = useRef<HTMLDivElement>(null);
   const previousResizingColumnIdRef = useRef<string | false>(false);
@@ -168,11 +170,12 @@ export function DatasetTable({
       columnWidths,
       isSelected: selection.has,
       toggleRow,
+      onCellExpand,
       isBuilding,
       pendingRowIds,
       flashingCells,
     }),
-    [tableRows, dataset.columns, columnWidths, selection.has, toggleRow, isBuilding, pendingRowIds, flashingCells],
+    [tableRows, dataset.columns, columnWidths, selection.has, toggleRow, onCellExpand, isBuilding, pendingRowIds, flashingCells],
   );
 
   return (
diff --git a/frontend/components/table/types.ts b/frontend/components/table/types.ts
index 5fc380c..6eff009 100644
--- a/frontend/components/table/types.ts
+++ b/frontend/components/table/types.ts
@@ -28,5 +28,6 @@ export interface DatasetRow {
   _id: string;
   _creationTime: number;
   data: Record<string, unknown>;
+  sources?: string[];
   updateStatus?: "pending";
 }