This dissertation investigates the sources of labor union bargaining power and the mechanisms that shape bargaining outcomes at the firm level in contemporary Chile. Set in 2004, the study examines variation in labor union formal bargaining outcomes within and across four holding corporations operating in the banking, manufacturing, retail and telecommunications sectors. It answers two main questions. What was the extent of variation in formal union collective bargaining outcomes within and across these companies? Why did some unions perform better than others? In addition, it offers an overview of the current conditions of Chilean collective bargaining and an explanation of the general decline of labor union bargaining power in that country since 1990. The main argument put forth is that union bargaining outcomes within and across companies are explained by variation in four independent variables: workplace conditions, labor market conditions, product market conditions and union organizational strength. These factors combined determine the particular balance of power between workers and employers during the bargaining process, affecting union and management bargaining interactions and ultimately shaping differences in individual unions' collective bargaining outcomes. The higher the value on each of the independent variables, the greater the bargaining leverage of the union and thus the better the bargaining outcome.