This paper examines the extent to which product market conditions limit the ability of trade unions to establish wage differentials over nonunion pay. The mean union pay differential is estimated to be of the order of 8 percent to 10 percent in establishments with some degree of product market power, but zero in establishments facing competitive product market conditions. When competitive conditions are faced, a preentry closed shop and high union coverage of the industry are both required. It is estimated that only 5 percent of the establishments in which there are significant differentials over nonunion pay face generally competitive product market conditions. Copyright 1990 by Royal Economic Society.