Why does the number of organizations of any given kind vary over time? Utilizing a diverse group of organizations including national labor unions, newspapers and newspaper publishers, brewing firms, life insurance companies, and banks, this book seeks to deepen and broaden the understanding of change in organizational populations by examining the dynamics of numbers of organizations in populations. Such an approach involves explaining the sources of growth and decline in the sum of organizations (what the authors call "density") over the histories of populations of organizations. The authors conclude their study by formulating a theory of density-dependent legitimation and competition.