Both replaced most historical float/table-based layout hacks, but they solve differently shaped problems.
| Flexbox | Grid | |
|---|---|---|
| Dimensionality | One-dimensional (a single row or column) | Two-dimensional (rows and columns together) |
| Item sizing | Content usually drives item size, container distributes leftover space | Container explicitly defines track sizes up front |
| Best for | Nav bars, button groups, centering one thing, any single line of items (wrapping or not) | Full page layouts, card galleries with aligned rows and columns, anything with a real two-axis template |
| Can they combine? | Yes — a Grid item can itself be a Flex container, and vice versa | Same |
A reasonable default: reach for Flexbox first for anything that's fundamentally a single line of items, and reach for Grid the moment you're actually defining a layout with meaningful structure across both axes. Using Grid for a simple one-line toolbar, or fighting Flexbox to fake a two-dimensional layout with wrapped rows that don't quite align, are both signs of reaching for the wrong tool.