Concurrent with the precipitous decline in private sector unionization over the past half century, there has been a shift in the type of work covered by unions. We take a skill-based approach to studying this shift, using data from the Current Population Survey combined with occupation-specific task requirements from the Dictionary of Occupational Titles and the Occupational Information Network. We partition skills into four groups based on two dimensions of task requirements: non-routine cognitive, non-routine manual, routine cognitive, and routine manual. For both men and women, private sector unionized jobs have changed to require more non-routine, cognitive skills and for women, less routine/manual skills. Union, non-union skill differences have grown, with unionized jobs requiring relatively more non-routine cognitive skill for both groups but also relatively more routine skills. We decompose these skill changes into: (1) changes in skills within an occupation, (2) changes in worker concentration across existing occupations, and (3) changes to the occupational mix from entry and exit. Most of the skill changes we document are driven by the second two forces. Finally, we discuss how this evidence can be reconciled with a model of skill-biased technological change that explicitly accounts for the institutional framework surrounding collective bargaining.