MoodScope is a first-of-its-kind smartphone software system that learns the mood of its user based on how the smartphone is used. While commonly available sensors on smartphones measure physical properties, MoodScope is a sensor that measures an important mental state of the user and brings mood as an important context into context-aware computing. We design MoodScope using a formative study with 32 participants and collect mood journals and usage data from them over two months. Through the study, we find that by analyzing communication history and application usage patterns, we can statistically infer a user’s daily mood average with 93% accuracy after a two-month training period. To a lesser extent, we can also estimate Sudden Mood Change events with reasonable accuracy (74%). Motivated by these results, we build a service, MoodScope, which analyzes usage history to act as a sensor of the user’s mood. We provide a MoodScope API for developers to use our system to create mood-enabled applications and create and deploy sample applications.