A critical assessment is given of the phase equilibria of the AI-Cu system. In the first section, the stable equilibrium diagram is constructed. In the second section, metastable and constrained equilibria are considered. These include metastable solvus curves of the AI-rich side of the diagram; on the Cu-rich side, transformations involving the metastable bcc phase include the order-disorder transition, metastable invariant reactions, and martensitic and massive transformation. In the third section, crystal structure and lattice-parameter data are reviewed. Finally, new calculations of selected phase equilibria are presented which are based on computer optimization of phase-diagram and thermochemical data. IMR/14;2