Gambling is a common human behavior. People who gamble encounter physicians. At times, gambling can produce adverse consequences for the player. Persistence of gambling despite adverse consequences can be called problem gambling. Roughly 1% of adults and 3% of adolescents exhibit signs of a medical disorder defined as pathological gambling. In many areas, including Wisconsin, gambling is increasing in legality, availability, and prevalence, and with it, pathological gambling is increasing in prevalence. This paper provides a descriptive review of gambling behaviors and the condition of pathological gambling, in the hope of increasing the awareness of practicing physicians, medical educators and researchers, and public policy makers, about what is known about these behaviors and the features of gambling illness that may present in a clinical setting. A companion paper focuses on the opportunities for constructive clinical activity by physicians regarding gambling problems.