One possible reason for ambiguous findings regarding changes in work skills is the way work skills are defined and measured. Five conceptual bases for defining work skills are illustrated in the research literature: abilities of individuals, behavior requirements of jobs, ability requirements of jobs, behaviors exhibited at work, and abilities exercised or used at work. Outside of the field of psychology, issues about the dimensions of skills have not received great attention. No universal taxonomy for defining the dimensions of work skills exists. Issues of measurement differ depending on whether the focus is on measuring skills of individuals or measuring skill requirements of jobs. Much interest in work skills concerns issues of change. Existing evidence, based on both large-scale assessments of skill changes and case studies, yields no firm conclusions about whether skill requirements of jobs are changing and, if so, in what ways. One way to further understanding of skill changes is to understand how various forces interact to influence skill requirements. Steps to help improve understanding of work skills and how they change are as follows: (1) studies of job skills should be based on comprehensive measures of both the qualitative and quantitative dimensions of work skills; (2) the concepts and measures of work skills should be applied both to capabilities of individuals and requirements of jobs; and (3) future research should attempt to develop a more complete model of the forces affecting job skill requirements. (Contains 77 references.) (YLB)