'Homology' as a concept became increasingly elusive during the course of the 20th century. The central debates and controversies concern both fundamental definitions and the nature of the criteria by which homology is judged. Attempts to move away from comparative morphology to ideas based on developmental pathways have tended to founder on the fact that developmental pathways evolve and that similar cells or tissues or structures in animals will often have different developmental origins. The use of information about conserved molecules in seemingly conserved developmental processes has also proven controversial.

In molecular biology, the use of the term 'homology' has given rise to more debate, although here the issue seems to involve primarily the criteria for assessing whether parts of genes are the same because of shared descent or for other reasons. The contributions to the book explore these topics systematically. There are chapters on the historical development of the concept of homology and its use in population studies. Other chapters deal with issues of homology in morphological and developmental studies, in behavioural studies, and especially in studies at the level of molecular genetics.