A model of density dependence is proposed to explain regularities in the growth and decline of organizational populations. Density -the number of organizations is assumed to be a function of the social processes of legitimation and competition. At low density, the model predicts that the legitimation process will dominate and will lead to high organizational founding rates and low organizational mortality rates. At high levels of density, competition will dominate, and consequently founding rates will decline and mortality rates will rise. The model is tested with hazard function models using data from nine newspaper populations spanning the 19th and 20th centuries and covering over 5200 newspapers. Analysis offounding rates provides strong empirical support for the model. Analysis of mortality rates reveals that the model holds for large populations.