Experimental design and methods: This is a two-group observational study. Each participant will be assessed in one main study visit and 5 additional visits for the EMA/GMA component of the study. The two main groups will be: (1) HIV+ participants, and (2) HIV- participants who are within the social network of HIV+ participants. Each group will be roughly matched for socioeconomic status and neighborhoods of residence. Each participant will attend a 3-5-hour initial study visit, a 1-hour training session for the EMA/GMA component, and four weekly 30-minute EMA/GMA compliance visits. The study visit will include a questionnaire component and a biological-sample component. Questionnaires will cover substance use, stress, social stability (including healthcare access/utilization, and neighborhood environment), physical and mental health (including quality of life, HIV, and mood). The biological component will consist of samples of urine (for drugs of abuse and cotinine), and breath (for alcohol and carbon monoxide). Blood for genetics will also be obtained at Visit 1. The primary outcome measures will be activity spaces as assessed with EMA and GMA, HIV status, knowledge of HIV-related healthcare and likelihood of seeking care, as well as risk/transmission behavior. Secondary outcome measures will include: biological assessments of drug use; medical, psychiatric, social determinants of health; impulsivity and decision-making tendencies. Participants will carry a smartphone for up to 4 weeks after completing Visit 1. Event-triggered entries will be initiated by participants (1) each time that they use a drug and/or alcohol, and (2) each time they engage in sexual activities with a partner. Participants will also make 5 random-signal-triggered recordings per day and one brief end of day recording.