The purpose of this research is to identify major factors that can be used to explain and predict the process of growth in union membership as represented by union victories in certification elections. The emphasis of this research is on organization and social-structural level factors. The logistic regression procedure reveals that organization level variables are most significant in explaining union victories in certification elections. Among the organization level variables, Unit Size, as defined by the NLRB, is the most significant variable in each year of the study and across all industrial classifications.