To better understand the circumstances, motivations, and social environments of injection initiation later in life (after turning 30 years old), RTI International conducted a quantitative and qualitative study of late initiation to injection drugs. The study had the following three aims: (1) To explore attributes of drugs and drug injection among late initiates using life course theory (LCT) constructs; (2) To describe individual characteristics, drug use histories, and key life events of late initiates as compared with typical initiates using LCT constructs; and (3) To explore the interpersonal, social, and structural environments of late initiates compared with typical initiates using LCT constructs.