Hybrid zones - geographical areas in which the hybrids of two races are found - have attracted the attention of evolutionary biologists for many years, both because they are windows on the evolutionary process and because the patterns of animals and plant variation seen in hybrid zones do not fit traditional classification schemes of taxonomists. Hybrid zones provide insights into the nature of the species, the way barriers to gene exchange function, the genetic basis of those barriers, the dynamics of the speciation process. Hybrid Zones and the Evolutionary Process synthesizes the extensive research literature and points to new directions in research.