Mental health peer specialists are individuals with serious mental illnesses who receive training to use their lived experiences to help others with serious mental illnesses in clinical settings. This Open Forum discusses the state of the research for mental health peer specialists and suggests a research agenda to advance the field. Studies have suggested that peer specialists vary widely in their roles, settings, and theoretical orientations. Theories of action have been proposed, but none have been tested. Outcome studies have shown benefits of peer specialists; however, many studies have methodological shortcomings. Qualitative descriptions of peer specialists are plentiful but lack grounding in implementation science frameworks. A research agenda advancing the field could include empirically testing theoretical mechanisms of peer specialists, developing a measure of peer specialist fidelity, conducting more rigorous outcomes studies, involving peer specialists in executing the research, and assessing various factors that influence implementing peer specialist services and testing strategies that could address those factors.