Selective and conformal heteroepitaxy of GaAs on silicon using a close-spaced vapor transport (CSVT) process based on a reversible water-vapor transport reaction is described here. In selective epitaxy, a GaAs him is grown preferentially through openings defined in an oxide mask which coats a silicon wafer. For conformal epitaxy, a simple CSVT process has been demonstrated for epitaxial lateral overgrowth of submicron-thick GaAs films in cavities formed on (1 1 1) silicon substrates. Details of a conformal GaAs-on-silicon CSVT technique and experimental results showing its feasibility are presented.