Everyone has suffered from the unpleasant symptoms (i.e., runny nose and fever) of an upper respiratory tract infection (URTI). These URTIs are commonly referred to as colds and are caused by more than 200 different viruses (16). Currently, URTIs are the most common types of infections worldwide, and the average adult suffers from two to five colds a year (10, 25). Although colds are not usually life threatening in healthy individuals, colds have many negative consequences because of in- creased health-care costs and lost days from work, school, and exercise training (12). Because of the high occurrence of colds, these illnesses present a real concern to the health of athletes and the general population. Since exercise and other stresses (e.g., emotional stress, loss of sleep, etc.) are known to influence the immune system, it is important to understand the relationship between physical activity and the immune system. Therefore, this chapter is designed to introduce the primary components of the immune system and to discuss how exercise training affects this important homeostatic system. We begin with a brief overview of how the immune system works to prevent infection.