Black women have the second highest rate of all new HIV infections in the U.S. after men who have sex with men, and they account for 69% of all HIV diagnoses among women in the South (e.g., Georgia). The objective of the proposed research is to refine and test in-the-kNOW (Novel approaches to Optimizing Women's Health), an evidence-based and theory-driven mobile application (app) specifically designed for PrEP-eligible Black women. This app responds to RFA-MH-21-150 Health Communication and Services Research priorities by incorporating portable, culturally- and contextually-relevant messaging strategies. Our definition of PrEP-eligible is based on CDC guidance for Black women, namely women who live in high HIV prevalence areas (specifically Ending the HIV Epidemic jurisdictions in Georgia: Fulton, Dekalb, Gwinnett, or Cobb counties). In our preliminary study, participants endorsed our mobile app intervention, in-the-kNOW, and expressed a desire for a fully developed and integrated version. Guided by Community-based Participatory Research (CBPR) principles and the Social Cognitive Theory of Mass Communication (SCT-MC), we will address the PrEP health communication gap with the in-the-Know app, which offers personalized messaging to promote HIV preventive behaviors among Black women in the South. We hypothesize that the in-the-kNOW intervention and the control condition (e.g. referral to the CDC's online HIV risk reduction tools) will be feasible to Black women. In a sample of PrEP-eligible Black women (ages 18-44), we propose the following aims: Aim 1. Refine the in-the-kNOW mobile app for PrEP-eligible Black women. Using a human-centered design approach 27, we will conduct a single-arm technology usability pretest to optimize usability, technical functionality, and performance of the app, among n = 10 PrEP-eligible Black women. The mobile app will be refined per pretest feedback. Aim 2a. Evaluate the feasibility, acceptability, and usability of the refined in-the-kNOW mobile app. We will employ experimental (n = 20) and control (n = 20) conditions to determine the feasibility and acceptability (retention rates and satisfaction) of both the app and the control condition (e.g., referral to the CDC's online HIV risk reduction tools). Aim 2b. Determine feasibility of collecting secondary outcome measures (e.g. PrEP Stigma) for a R01.