Mechanisms Underlying Class Size Effects explores how class size interacts with variables found in a young child''s learning environment. The study begins with the development of a systems-based, causal model that includes clusters of parent, child, teacher, and classroom variables derived from the literature. Multiple Regressions were then used to test the ‘fit’ between the data collected for the study and the causal model. The results illustrate the pervasive nature of class size effects. Not only does class size explain some variance in a number of variables contained within the causal model as predicted, class size was found to subsequently interact with those same variables to modify other variables. This trajectory of interactions suggests that class size effects are cumulative. Moreover, class size effects were found to be modified by other variables in the model. Given the small sample size (117 children, their parents and their teachers), findings from the study should be regarded as a preliminary test of the model.