This project addresses depression, a mood disorder that impacts millions of American's each year and is one of the leading causes of disability. Depression can be hard to diagnois. It can also be difficult to tell if people respond favorably to treatment. Our research focuses on building an intelligent system that can detect when someone is experiencing the onset or recurrence of depression (transitioning from healthy to unhealthy) or when someone experiences remission from depression (transition from unhealthy to healthy). Our system, Big Black Dog, monitors people's social behaviors, activity level, and sleep patterns to infer when a transition is taking place.