Undertaking a trip to Mars will require unparalleled time and effort for astronauts. Crews will live and work together for 2 to 3 years in an isolated and confined extreme environment with varying levels of taskload and incessant risk of vehicle malfunctions and failures. The compatibility and performance of the crew are therefore crucial to these longduration exploration missions (LDEMs). In particular, the function and coordination of these small teams will be influenced by the behavioral and social characteristics of the individual members, especially their personality type. This necessitates a special focus on crew selection and composition for LDEMs. Planning for optimal team performance begins at the selection process to become astronaut candidates where applicants who make it to later stages undergo an extensive screening process as well as comprehensive psychological and physiological testing. Currently, personality testing is used during these processes to "select out" certain individuals from the astronaut candidate program who may be ill-equipped for the astronaut corps. However, personality's integral role in job performance and team performance suggests a need for "selecting in" certain personality traits, such as agreeableness and conscientiousness. Although the initial selection process relies on a standardized, multi-faceted approach, the procedures for mission assignment from the pool of astronaut candidates are seemingly less structured, given available documentation, and performed by the Astronaut Office. In this paper, we argue that selecting individuals for LDEMs will require more emphasis on teambased considerations than just individual-level criteria (e.g., technical skills, experience, etc.). For instance, crew compatibility on an interpersonal level will need to be a major factor in order to ensure optimal communication and coordination within the team. A primary consideration in forming the optimal mixture of individuals for a team is the specific personality combinations of its members. The wrong mixture can produce a number of deleterious effects including, but not limited to: subgroup formation, conflicts over values, and weakening of team cohesion. To address these issues, this paper: a) summarizes current astronaut selection procedures and small-team selection procedures, b) summarizes existing literature on personality composition for small-team performance, c) makes recommendations about the inclusion of personality metrics for both astronaut and crew selection from existing research, and d) suggests further research on team selection and composition via space analogs.