HIV-related sexual risk behavior is commonly ascertained by collecting retrospectively enumerated sexual activities that span 3-month or 6-month periods. This approach is problematic because of recall bias, potentially leading to underreporting of key behaviors. We propose to design, and determine feasibility and acceptability, and validity of a novel sexual health-oriented mobile health (mHealth) application (app) to assist with daily reporting of sexual activities and thereby eliminate the problems associated with recall bias. To accomplish this aim, we will conduct a focus group with iterative versioning of the app to complete a participant-friendly app. We will then conduct a pilot feasibility and acceptability study by recruiting 30 men who have sex with men (MSM) to use the app for three months and compare the data obtained through the app with the data obtained through the standard retrospective recall method. This mHealth app has the potential to be a "game changer" regarding how data is collected for HIV-risk behaviors. Once designed and validated, the app will be available to other investigators and their research projects. Pilot data from this project will lead to a K-23 application to study the use of mhealth technology in HIV prevention.