Contents: (1) Introduction. (2) Closure quantities: Interferometry. Closure phase. Closure amplitude. Interpretation of closure phase and closure amplitude. (3) Aperture synthesis: Basic theory. Image restoration. (4) Internal calibration methods. (5) Hybrid-mapping methods: Method of Baldwin & Warner. Method of Readhead & Wilkinson. Method of Schwab and Cornwell & Wilkinson. Maximum entropy methods. Redundancy. Global fringe-fitting in VLBI. (6) Capabilities and limitations: Uniqueness and uv-plane coverage. Noise. Convergence. Closure errors. (7) The future.