Every day, people are reliant on smartphones to stay connected, organized, and entertained. Every year, smartphones are equipped with more processing power, digital storage, and sensors. As a result of their growing ubiquity and technical advances, there are now exciting opportunities for leveraging these devices in clinical health research, care, and intervention. Collectively, these opportunities are often referred to as the emergent “mobile health” or mHealth trend. Here, we briefly discuss 3 areas where smartphones can make a significant impact (ambulatory assessment, behavior monitoring, understanding and predicting outcomes), using a system that we have developed—Emotion Sense—as a case study. We also caution about some of the limitations of this technology.