mHealth denotes the use of mobile devices within a health care context. One type of mHealth that has gained increased popularity is the use of mobile applications (apps). Despite a plethora of apps that are commercially available, the efficacy or effectiveness of these apps is largely unknown. This article reviews the literature on the use of mental health mobile apps in a psychotherapy context. The review focuses on the efficacy or effectiveness and common features of mental health apps. At present, there is insufficient empirical support for any 1 particular app to be considered evidence-based. A number of methodological concerns among treatment outcome studies further complicate conclusions regarding efficacy and effectiveness. Nonetheless, preliminary results are promising and warrant further research. Apps included in this review were generally extensions of empirically supported treatments, primarily grounded in cognitiveAaAeAeAubehavioral therapy. Implications and clinic issues for practitioners are discussed. Given the current state of the research, clinicians may wish to consider cautiously incorporating apps as an adjunct to treatment or recommending apps to clients, but much is unknown including the possibility that in some circumstances, particular apps may prove to be iatrogenic for some clients.