In this chapter, the authors review the history of the Waterloo Longitudinal Project (WLP), the first longitudinal study (1980-1992) dedicated to the study of social withdrawal, its correlates, and consequences. Theories underlying the WLP are described, as are its empirical findings. Recent research from other labs that has extended the findings of the original WLP is briefly described. The authors' research that draws on the findings of WLP are noted as well. An underlying theme in this work is that relationships (and interactions) with parents and friends can serve as protective or exacerbating factors in the developmental course of social withdrawal and its concomitants (including social anxiety).