It is easy to imagine how positive psychology (the science of human flourishing) applies to people who are already doing well and wish to do better. It is less obvious how to apply positive psychology in negative contexts: Can positive psychology concepts and strategies help people flourish in the face of mental illness, trauma, and loss? The current investigation presents findings from three randomized trials of interventions informed by positive and clinical psychology, which aim to help people survive and thrive in the face of highly challenging circumstances: depression, mixed traumatic and adverse events, and bereavement. Chapter 1 summarizes the findings of a randomized controlled trial evaluating a smartphone-based/web-based application (app) that integrates clinical and positive psychology strategies with game mechanics in order to alleviate depression symptoms. Results indicated that the app reduced symptoms of depression (in comparison to a waiting list control) and that there were no significant differences between two versions of the app. Chapter 2 summarizes the findings of a randomized controlled trial evaluating an online writing-based intervention aimed at fostering posttraumatic growth (PTG) after adverse events. This intervention, called prospective writing, prompts participants to seek new doors opening in their lives in the wake of loss and trauma. Results indicated that prospective writing fostered PTG for people with recent and long-ago trauma/loss, and mediation analyses suggested that attending to new possibilities was indeed the mechanism for this change. Chapter 3 describes the creation and initial testing of a group-format psychosocial intervention aimed at fostering PTG. Acceptability and feasibility analyses of the data (from an ongoing randomized trial) indicated that bereaved adult participants found this intervention helpful, engaging, inoffensive, and not overly upsetting; that they appreciated diverse intervention modules; and that they would recommend the intervention to other bereaved people. Collectively, these findings underscore the usefulness of positive psychology in negative contexts and suggest further research into intervention strategies that can help suffering people to not only survive but also thrive in the wake of adversity.