Any angular momentum control device requires a precise and reliable rotational degree of freedom to spin a rotor, or to gimbal a spinning rotor. This is most commonly achieved through the use of ball bearings. Due to the extremely high numbers of failures and unexplained anomalies of ball bearing in spaceflight mechanisms, they have attained a mystique of being very difficult to implement, and therefore subjected to highly sensitive and specific design rules, delicate handling practices, stringent cleanliness requirements, and extremely conservative lifetime factors. While such best practices are always advised for the objectives of launching high reliability equipment into space, numerous examples of unexpected bearing problems in space have been encountered even after diligent adherence to these requirements and guidelines (Shapiro et al., Space mechanisms lessons learned study. NASA technical memorandum 107046. Tech. rep., 1995). So, in this chapter we discuss the basic design variables and tradeoffs for bearings used in momentum control devices. The goal is to impart a broad understanding of the most important considerations of ball bearings when selecting or reviewing the design of a component which is used in a momentum control system, or when one is involved in unexpected anomalies or failures of ball bearings.