Mobile Messaging Intervention to Present New HIV Prevention Options for MSM (MMI4MSM) is a randomized control trial taking place in Atlanta, New York City, and Detroit evaluating the acceptability, use, and effectiveness of a smartphone-based platform for delivering sexual health and prevention messages to men who have sex with men. The study will evaluate the use and efficacy of mobile-messaging platforms as a public health strategy for improving sexual health outcome measures among MSM by determining whether exposure to the message-delivery platform results in improvements in participants’ self-reported sexual health and prevention behaviors, beliefs, and attitudes.