These two volumes consist of chapters written by students and colleagues of W.K. Estes. The books’ contributors – themselves eminent figures in the field – reflect on Estes’ sweeping contributions to mathematical as well as cognitive and experimental psychology. As indicated by their titles, Volume I features mathematical and theoretical essays, and Volume II presents cognitive and experimental essays. Both volumes contain insightful literature reviews as well as descriptions of exciting new theoretical and empirical advances. Many of the essays also incorporate personal reminiscences reflecting the authors’ fond affection for their illustrious mentor.