Traditionally, leadership research relies on tools such as observations and surveys to characterize leaders’ traits and behaviors, followers’ reactions, and leader–follower interactions and coordination to understand the leadership process. Recent development in biological and physiological assessments has offered leadership scholars additional ways to measure processes in leaders and followers. These methods can be used to capture subtle processes of different biological systems, such as activities in the brain, cardiovascular, and endocrine systems of leaders and followers. Such processes may not only be outside individuals’ awareness, but also have limited outwardly visible signs. Despite this, they provide additional insights into understanding how leaders exercise effective influence; how followers perceive, share, and respond to messages from leaders; and how leaders and followers reciprocally affect each other. In this chapter, the authors first introduce biological and physiological sensors, and discuss their applications in the leadership research. Next, they discuss various biological and physiological measures and their advantages over the traditional measurement tools typically used in leadership research. Third, they review the extent to which these biological and physiological measures already have been adopted by leadership researchers. Finally, recommendations and future directions are presented for leadership scholars who are interested in utilizing these measures.