In this report, we introduce Forrester's App Engagement Index, a framework that evaluates media, communications, and entertainment apps based on key behavioral metrics: popularity, audience commitment, frequency of access, and time spent using it. The App Engagement Index scores, ranks, and groups individual apps and app categories based on smartphone behavioral tracking data to create insight into what makes an engaging app. Marketing leaders can use the Index to help decide which apps makes appropriate partners, while those creating media and communication apps can use it to benchmark their own performance.