Thin film deposition is a process complicated by chemical and physical phenomena that take place on different time and length scales. Its correct description requires the development of multiscale models. Here we present the multiscale approach we developed to study the epitaxial chemical vapour deposition (CVD) and etching processes. The model is designed to describe the atomic scale with quantum chemistry, the morphology evolution at the mesoscale, with a Kinetic Monte Carlo (KMC) model utilizing parameters either taken from the literature or computed with the atomic scale model, and the fluid dynamic and local composition in the reactor through the solution of mass, energy and momentum conservation equations with the finite element method. The multiscale approach was used to investigate the gas-phase chemistry active during the MOCVD of II-VI semiconductors, the morphology evolution during the epitaxial deposition of Si, and the fluid dynamic and gas-phase composition in a plasma-etching reactor. (C) 2005 Elsevier Ltd. All rights reserved.