Digital image processing is an essential step in the determination of macromolecular structures by electron microscopy. Centrally important procedures are the averaging of many images of the subunit to improve the signal, the correction for various transfer functions, and the generation of a three-dimensional map from a set of two-dimensional projections. The detailed way in which these computational procedures are best carried out depends on the symmetry of the object and the type of specimen preparation. Over many years a large set of programs has been written by various members of the Laboratory of Molecular Biology for processing images of two-dimensional crystals and of particles with helical or icosahedral symmetry. The philosophy has been to write stand-alone programs and the whole system is given coherence by the adoption of standard formats for the storage and interchange of different kinds of data. This paper describes the current state of the programs.