Many mood tracking apps are available on smartphone app stores, but little is known about their features and their users' experiences. To investigate commercially available mood tracking apps, we conducted an in-depth feature analysis of 32 apps, and performed a qualitative analysis of a set of user reviews. Informed by a widely adopted personal informatics framework, we conducted a feature analysis to investigate how these apps support four stages of selftracking: preparation, collection, reflection, and action; and found that mood tracking apps offer many features for the collection and reflection stages, but lack adequate support for the preparation and action stages. Through the qualitative analysis of user reviews, we found that users utilize mood tracking to learn about their mood patterns, improve their mood, and self-manage their mental illnesses. In this paper, we present our findings and discuss implications for mobile apps designed to enhance emotional wellness.