In the efforts to disentangle life stage from other factors with which it is confounded, the conceptual meaning and developmental role of both history and cohort have been ignored. This paper reconsiders the nature of cohort, a conceptual formulation of it, and three problems in its application (cohort boundaries, distinctive cohort experiences, and differential cohort effects). While cohort is an index of many discrete variables that can be specified in its place, its subtler patterns are less amenable to operationalization and substitution. Hence, there are limits to the dispensability of this sensitizing concept.