Reflection seismology seeks to determine the structure of the earth from seismic records obtained at the surface. The processing of these data by digital computers is aimed at rendering them more comprehensible geologically. Seismic migration is one of these processes. Its purpose is to "migrate" the recorded events to their correct spatial positions by backward projection or depropagation based on wave theoretical considerations. During the last 15 years several methods have appeared on the scene. The purpose of this paper is to provide an overview of the major advances in this field. Migration methods examined here fall in three major categories: 1) integral solutions, 2) depth extrapolation methods, and 3) time extrapolation methods. Within these categories, the pertinent equations and numerical techniques are discussed in some detail. The topic of migration before stacking is treated separately with an outline of two different approaches to this important problem.