Humor can be a powerful therapeutic tool in clinical social work: It creates a layer of connection between clinician and client, can strengthen the therapeutic bond and provides a gateway to change on cognitive, emotional, and biological levels. It can help restore a sense of playfulness, lightness, and fun—innate qualities often lost as a result of adverse early experiences, including insecure attachment and trauma. The first paper of this two-paper theoretical dissertation reviews facets of humor development as an integrated social, cognitive, and emotional system that starts in infancy, with the objective of providing clinicians with a theoretical basis for integrating humor into therapy. Humor is examined through the play system as a component of attachment theory, mentalization, and the cognitive/emotional appreciation of incongruities. Theory is linked to practice through clinical vignettes. The second paper focuses more deeply on the clinical application of humor in clinical social work and the ways in which humor can be utilized by clinicians as a therapeutic tool. It addresses therapists’ use of self, categories of humor, the importance of increasing mentalization skills to heighten awareness and timing in humorous interjections, the broaden and build theory of positive emotions, recent studies correlating humor, well-being and resiliency, and caveats about using humor.