Originally published in 1986, this volume was the result of a conference in honor of the 65th birthday of the late Kenneth MacCorquodale, an exceptionally eloquent spokesman for the field of experimental analysis of behaviour at the time. The present volume grew directly out of the issues raised by MacCorquodale and Meehl in their Excursis: The Response Concept paper and which MacCorquodale posed so often when he taught. It is a fitting tribute to the man on his 65th birthday that a group of scholars whom he held in the highest regard convened in one place to think out loud about two of the thorniest problems facing behavioral science, namely, the nature of the units of analysis of the subject matter and the mechanisms responsible for their integration.