The topic of gratitude is receiving increasing attention in psychology because it encompasses promising developments: the significance of positive emotions in interpersonal relationships and a focus on resources. Many studies demonstrate the high relevance of gratitude for mental health. In particular, positive psychology emphasizes the health-promoting potential of a grateful and appreciative attitude in life. This book presents, for the first time in the German-speaking world, the current state of knowledge surrounding gratitude. It introduces diagnostic methods for measuring gratitude and describes how gratitude can be fostered within the framework of psychotherapy. The internet-based gratitude training developed by the authors has proven effective in several studies in reducing worries, rumination, and depression. All working materials are presented for the first time in this book as worksheets and information sheets, which can be used in excerpts or as a structured program. Copy-friendly working materials, including three audio tracks with imagination exercises, are included on the accompanying CD-ROM. However, the topic of gratitude also has its downsides and challenges. For instance, it is essential for psychotherapists to respond appropriately to gratitude-related situations in the therapeutic relationship. It is also important to understand the indications and side effects of gratitude interventions. The authors illustrate, through numerous case examples, how psychotherapists can reflectively engage with the topic of gratitude and practically expand their options for action.