Human factors addresses humans in their active and interactive capacities. The importance of human factors has risen sharply with the increased activity and responsibility of space crews and will culminate with missions of exploration. Human factors requirements are highly dependent on the characteristics and conditions of the mission, and an explication of human factors requirements must begin with a detailed mission analysis. A framework in which human factors issues could be considered is outlined, and candidate areas for early investigation are proposed. Approaches needed to transition research results to mission planners and implementers are discussed.