—Computational models of development aim to describe the mechanisms that underlie the acquisition of new skills or the emergence of new capabilities. The strength of a model is judged by both its ability to explain the phenomena in question as well as its ability to generate new hypotheses, generalize to new situations, and provide a unifying conceptual framework. Although often constructed using traditional engineering methodologies, evaluating the performance of a computational model of development in terms of traditional perspectives, however, is a flawed approach. This paper addresses the fundamental issues that confound quantitative analysis of computational models of developmental systems. In particular we focus on the following recommendations: 1) don’t equate the success of a developmental model with its peak performance at some task; 2) don’t employ purely subjective or qualitative measures of model fitness; and 3) don’t hide or reject variation as found in the computa...