Objective. To use Smartphones (PDAs) to measure stress and drug use; Global Positioning System (GPS) units to assess geographical location; biological samples for genetic testing; Daysimeters and Dimesimeters to assess circadian rhythm; AutoSense to collect ambulatory physiological and activity data in real time; and to assess the feasibility and acceptability of mobile HIV/STD Risk Reduction (HIVRR) delivered via PDA in real time.