This chapter examines the ancient Stoic theory of the physical composition of pneuma, how its composition relates to pneuma’s many causal roles in Stoic philosophy, and to what extent each of the first three leaders of the Stoic school accepted the claim that pneuma pervades the cosmos. I argue that pneuma is a compound of fire and air. Furthermore, many functions of pneuma can be reduced to the functions of these elements. Finally, it is likely that each of the early Stoics posited a pervasive cosmic pneuma. This paper also explores the nature of pneuma’s tensile movement and offers an account of fire and air’s roles in causing its motion.