Guide to mastering CSS Grid for complex, responsive two-dimensional layouts: understand containers, items, tracks, and cells; define structure with grid-template-rows/columns; position via grid-row/column or auto-placement; use template areas, implicit grids, and subgrids; follow fr-based, simple naming best practices; apply it to an e-commerce dashboard (12x6 grid, KPI cards, subgrid charts), and test across devices.