This monograph is the written version of a series of talks delivered as recent MacEachran Lectures at the University of Alberta. The informal style of the lectures, and the inclusion of a relatively large number of figures, has been preserved in order to keep the monograph faithful to the concept of an individual attempting to integrate his own research into a reasonably coherent framework. Although the volume is very much a personal account of one individual's perspective, the studies reported are naturally a product of many collaborations as well as inspirations from colleagues. The fundamental issue addressed is how adult age differences in fluid or process aspects of cognitive functioning are to be explained. Several potential mediators are considered, with most of the emphasis devoted to the investigation of working memory and processing speed as variables mediating relations between age and cognition.