After introducing cannabis as the most prevalent illegal psychoactive substance used by adolescents and adults in the United States and other countries, the authors present the dramatic increase in cannabis use in the United States, starting in the early 1960’s. Following a discussion of three major waves of new, younger and younger cannabis users during the past 40 years, the authors maintain that the popular perception that cannabis is a low risk drug is not born out by the epidemiological evidence that has been accumulating for the past 20 years. Through the presentation of figures and tables, the authors maintain that 1 percent of the population in the United States is estimated to meet the criteria for current cannabis dependence, including 2.6 percent of adolescents aged 12- to 17-years-old, 3.5 percent of young adults aged 18- to 25-years-old, and .4 percent of adults 26-years-old and older. Addressing the negative characteristics of cannabis use, the authors detail a wide variety of psychiatric conditions, health and school problems, and criminal offenses related to marijuana usage. Discussing cannabis users who turn to public health practitioners for treatment, the authors present a variety of demographic and clinical characteristics of 86,753 adolescents and 298,796 adults who were admitted to public outpatient treatments in 1998. After arguing that regular cannabis use is associated with a wide range of health, emotional, behavioral, social, and legal problems, the authors conclude that there is an unmet need for cannabis treatment in the United States and in other nations. Tables, references