Unions in the U.S. Private Sector have declined dramatically over the past half century. In the context of a two-sided model of selection into union jobs, I decompose the decline of unions into components due to changes in worker demand and changes in the supply of union jobs relative to demand. I use small-scale survey data collected at several points over the past 40+ years related to worker demand for union representation to identify movements in supply and demand. I conclude that most of the decline of U is due to a decline in the relative supply of union jobs. I then use data from the CPS to examine the role of right-to-work laws in encouraging free-riding and reducing the relative supply of union jobs.