ABSTRACT African adolescent girls and young women (AGYW) face high rates of HIV and common mental disorders. HIV pre-exposure prophylaxis (PrEP) is a highly effective HIV prevention approach which is being scaled up in South Africa. However, symptoms of common mental disorders are significant barriers to PrEP use. Effective mental health treatment, such as problem-solving therapy (PST), has the potential to be integrated with PrEP delivery to address mental health challenges and modifiable barriers to PrEP use and improve PrEP adherence for AGYW. The Friendship Bench is a promising evidence-based PST intervention option for AGYW in PrEP settings given that it was developed for delivery in busy HIV clinics, is currently being scaled-up in South Africa, and preliminary data suggests it can be adapted to improve HIV treatment adherence among African women. Research is needed to evaluate its effectiveness on mental health and PrEP adherence outcomes and identify implementation strategies for AGYW in PrEP delivery settings. In a prior K99 study (K99 MH123369), Dr. Velloza and her team conducted human-centered design work to understand needs around integrated mental health and PrEP intervention delivery and to adapt the Friendship Bench intervention and implementation strategies. The overall goal of the current R00 study (R00 MH123369) is to test and optimize the adapted Friendship Bench for African AGYW in PrEP delivery. The Specific Aims are to conduct a hybrid Type II implementation-effectiveness trial to assess preliminary effectiveness of Youth Friendship Bench SA on PrEP adherence and mental health while optimizing implementation for cost constraints in PrEP delivery (Aim 1) and identify themes around intervention acceptability, appropriateness, and feasibility (Aim 2). In preparation for Aim 2 of this work, we conducted a systematic review of acceptability metrics and met with key leaders in the HIV and implementation science fields to discuss measurement approaches for these early implementation outcomes. We found significant gaps in high quality acceptability, appropriateness, and feasibility metrics and a lack of standardization in how these constructs are operationalized in HIV-related research specifically and the field of implementation science more broadly. These gaps have prevented us from conducting Aim 2 data collection and analyses. This 12-month supplement application proposes to enhance the existing parent project by identifying and validating acceptability, appropriateness, and feasibility metrics to inform Aim 2 of the parent study. We propose to leverage our current R00 study to: 1) develop a toolkit of “best practice” acceptability, appropriateness, and feasibility measures to inform Aim 2 of the parent R00 and harmonize implementation science efforts in the HIV prevention field; and 2) validate acceptability, appropriateness, and feasibility measures in the context of our PrEP implementation trial in Johannesburg. We are well-p...