Grid and Flexbox both solve layout problems that older CSS approaches (floats, inline-block, absolute positioning) handled clumsily, but they aren't interchangeable, they were designed for different shapes of problem. The single clearest distinguishing question is: are you arranging items along one axis, or do you need control over both rows and columns simultaneously?
Flexbox: one dimension, content-driven
Flexbox lays items out along a single axis, either a row or a column, and excels when the content itself should determine sizing, items grow, shrink, and wrap based on available space and their own natural size. A navigation bar with a logo on the left and links on the right, a row of buttons that should space themselves evenly, or a column of form fields that should stack and fill available height are all classic one-dimensional problems where Flexbox is the natural, minimal-code solution.
Flexbox properties like `justify-content` and `align-items` control alignment along and across the single axis, and `flex-wrap` lets items spill onto a new line when they run out of room, but Flexbox doesn't let you align items across multiple rows into consistent columns, once it wraps, each row is independent. That's precisely the gap Grid fills.
Grid: two dimensions, layout-driven
Grid lets you define both rows and columns explicitly and place items into that structure with real control over both dimensions at once, items in one row can align precisely with items in another row, something Flexbox structurally can't guarantee once wrapping is involved. A full page layout with a header, sidebar, main content, and footer, or a photo gallery where every card needs to line up in a clean grid regardless of content length, are textbook two-dimensional problems.
Grid also supports naming areas (`grid-template-areas`) so a layout's structure can be described almost like a diagram directly in CSS, and it handles responsive breakpoints elegantly with `auto-fit`/`auto-fill` and `minmax()`, letting a grid reflow its column count automatically as the viewport changes without writing separate rules for the item widths. Prototyping either approach visually with the CSS Grid Generator or CSS Flexbox Generator generates the exact CSS from a visual layout rather than requiring you to hand-write and mentally simulate the syntax.
In practice, most real interfaces use both
A typical page uses Grid for the overall page skeleton (header, sidebar, content, footer) and Flexbox inside individual components (a card's internal row of an icon, title, and button). Treating them as competitors misses the point, they're complementary tools operating at different structural levels of the same page, and reaching for the wrong one for a given problem (say, faking a two-dimensional grid with nested Flexbox rows) usually means more CSS and more fragile alignment than just using Grid where it fits naturally.