This protocol describes a 20-year prospective cohort study, gathering behavioral and psychosocial information on clients who access services of the Institute of HIV Research and Innovation (IHRI) and the Thai Red Cross AIDS Research Centre through the Thai Red Cross Anonymous Clinic, the Thai Red Cross Mobile Clinics, Affiliated Health Clinics, The gender and immunity clinic and Pribta Tangerine Polyclinic. Services provided through these sites include VCT, STD screening and treatment, anal and cervical Pap smears, general health checkups, sexual health education, and sexual health communication, gender-affirmative hormone therapy (GAHT) and risk-reduction via web boards and chat rooms. Behavioral and psychosocial information will be collected using validated questionnaires and from data collection forms routinely used when providing services in different service sites of IHRI and the TRC-ARC. The information will be collected anonymously at baseline and at follow-up visits, according to type of services each client receives. Clients at the Thai Red Cross Anonymous Clinic, Thai Red Cross Mobile Health Clinics, Affiliated Health Clinics and Pribta Tangerine Polyclinic will be given an identification number. Qualitative methods, including focus group discussions (FGD) and in-depth interviews (IDI) will be conducted with clients and healthcare providers to examine the facilitators, barriers, feasibility and acceptability of service implementation.