This paper clarifies the relationship between opportunity and mobility: first, by analyzing the ways opportunity differentials are created into three generic possibilities—through positional historical, and resource variation; and, second, by uncovering certain logical relations that exist between opportunity differentials and differences in the amount of career mobility individuals experience. The first point offers a useful classification for ordering and comparing diverse theoretical approaches to career mobility. The basic conclusion of the second point is that all differences in career mobility between groups of individuals are logically explained by differences in the opportunities to move to which they are exposed and, further, all differences in the career mobility of individuals are due to chance variation within the constraints of the logically true aggregate relationships.