Religion and spirituality (R/S) may shape individual's conceptualizations of, goals for, and experiences with behavioral health and treatment, coping, healing, growth, and empowerment. However, client R/S is often an underutilized facet of the environment in U.S. behavioral health care, due in part to a lack of training on how to effectively engage client R/S. To investigate the benefits of engaging client R/S in behavioral health care, I conducted a systematic search of the literature from the last 20 years on behavioral health engagement with client R/S, using three databases (APA PsycINFO, CINAHL Complete, and MEDLINE). This returned 82 unique results from which I systematically selected 12 studies for analysis by applying inclusion and exclusion criteria. Additionally, I identified several key informants and created an interview protocol. Interview transcript analysis can lead to a more nuanced understanding of factors related to engaging client R/S in behavioral health care. This review reveals that limited research - particularly, few empirical studies - from the past 20 years exists on engaging client R/S in behavioral health care. However, all findings on engagement with client R/S describe a variety of positive impacts. Additional studies, particularly on engaging client systems with forms of R/S besides Christianity, are needed to strengthen training in culturally responsive practice, which includes engagement with client R/S. Better engagement may improve service utilization and client outcomes.