The reasons a skill mix among health workers is important to health care systems were examined. The analysis was based on a review of studies conducted primarily in the United States. "Skill mix" was defined as the mix of posts, grades, or occupations in an organization and the combinations of activities or skills needed for each job within the organization. The literature reviewed included the following types of publications: reviews; meta-analyses; large-scale data surveys; single-site examinations of roles and mix in nursing and other nonmedical health professions; single-site examinations of role overlap between doctors and other health professionals; and studies on the introduction of new types of workers. Significant limitations to the current evidence on skill mix in the health workforce were discovered. Many published studies on the topic were merely descriptive accounts, and the usefulness of many of the studies that moved beyond mere descriptions was constrained by methodological weaknesses, lack of appropriate evaluations of quality/outcome and cost, and/or use of small samples. Despite these limitations, the evidence suggests that increased use of less qualified (cheaper) nursing staff will not be effective in all situations, although greater use of assistants has improved organizational efficiency in some areas. (French and Spanish abstracts are included. The bibliography lists 41 references.) (MN)