The Phylodynamics-targeted Partner service Models (P2M) project aims to guide and transform the rapidly evolving public health implementation of molecular HIV surveillance (MHS) based prevention interventions as a critical step towards HIV elimination. P2M seeks to study and optimize emerging new national guidance: The CDC's March 2017 recommendations of the HIV Cluster Guidance Working Group on health department implementation of MHS with clinical HIV resistance genotypes, now recommended at entry to care. These HIV-1 pol sequences can be used to identify ?molecular clusters? of persons with genetically similar HIV. The overarching goal of MHS is to identify persons within the risk environment. These persons are potential sources, or recipients, of new HIV transmissions, and defined as: 1) new HIV seropositives (including recent/acute infections); 2) previously known HIV seropositives with detectable viral load (in care or not); and 3) at-risk HIV seronegatives. The yield of these persons in the risk environment is an important metric of the usefulness of MHS-based partner services or other network recruitment procedures (ie 2-step) that will be examined in P2M. Thus, P2M aims to rigorously test the hypotheses that: (a) analyzing molecular cluster data closer to time of HIV diagnosis will increase the yield of risk environment members; and (b) targeted prevention using advanced partner and network service approaches our team has innovated will also increase yield and improve effectiveness of public health combination interventions. It is not feasible, however, to conduct a series of randomized trials to test these hypotheses. Modeling approaches, however, are best suited for providing guidance when multiple approaches and conceivable conditions exist, the contexts do not lend themselves to research trials, and where multiple downstream outcomes are possible. For P2M, members of this multi- institutional investigative team (from Houston and Chicago) will build upon existing HIV TRACE collaborations, observational cohorts of populations most impacted by HIV and an existing Agent-Based HIV transmission model (ABM) to guide and prioritize interventions. Accordingly, we aim to: Aim 1) Determine the relative effectiveness of partner services targeted to HIV TRACE derived molecular clusters compared to partner services as currently delivered without molecular cluster targeting (standard of care) to yield persons in the risk environment. Aim 2) Optimize several cluster size and time thresholds by parameterizing an agent-based model with existing cohort data that best yield categories of individuals in the risk environment based upon: 1) time-to-molecular cluster identification; 2) molecular cluster number and size; and 3) real-time versus delayed partner services response to cluster growth. Aim 3a) Model relative effectiveness of combined molecular surveillance and next-generation partner services ? 2-step, 3-step and social network referral ? that best yield persons in the risk environment; Aim 3b) Explore simulated HIV prevention interventions (ie. PrEP) and their cost as they are modeled within real-world molecular cluster and partner/network service strategies.