Gordon Arthur Riley (1911–1985) is remembered for his pioneering work in the development of marine ecosystem models during the mid-20th century. Using models that were necessarily simple because of the limited understanding of plankton physiology at the time, as well as the fact that calculations had to be done by hand, Riley studied the processes that control plankton stocks, production, and nutrient cycling, notably at Georges Bank. His great achievement lay not so much in the simulation of plankton dynamics per se, but rather in bringing to the fore the concept of using modeling as a means of explaining and interpreting the dynamics of marine ecosystems. In this article, we examine Riley's approach and philosophy to ecosystem modeling, which we discuss in context of modern day approaches. In particular, we focus on his landmark paper describing a model study of the dynamics of phytoplankton production on Georges Bank (Riley, 1946: J. Mar. Res., 6, 54-73). After reconstructing the model, we show how Riley created new mathematical characterizations of the environmental dependencies of each process in the phytoplankton equation, and how these relate to modern day formulations. We then reproduce Riley's results and conduct further analyses and sensitivity tests which serve to illustrate Riley's conviction that mathematical models can provide clear, rational explanations for the observed temporal changes in ecosystems. Riley's methods and outlook are discussed in context of the ongoing debate about the merits of complex versus simple marine ecosystem models. Based on our analyses of Riley's model, as well as his own critiques, we argue that although recent decades have seen a proliferation of complex ecosystem models that are intended to reflect our expanded understanding, the doctrines proposed by Riley are no less relevant today. In particular, Riley noted that while increasing model complexity is generally desirable, it can only be done within the confines afforded by observational data and knowledge of the physiology and ecology of key species and their interactions.