Depression and insomnia are highly prevalent, impactful, and mutually reinforcing conditions. Despite this, understanding and intervening on depression and insomnia, jointly, can feel quixotic, a phenomenological “chicken-or-egg” paradox. The purpose of this book is to equip mental health professionals with the latest theory and research on depression, insomnia, and their co-occurrence. Using a cognitive behavioral framework, it also seeks to equip the reader with specific recommendations for conceptualizing, assessing, and treating depression, insomnia, and related cognitive, emotional, and behavioral processes in therapy.