In this chapter, we describe laughter’s essential nature by extracting and carefully analyzing each element within a new and universal definition: Laughter—a vocal affirmation of mutual vulnerability. Vulnerability. This concept is at the heart of the mutual vulnerability theory of laughter and describes a trait (physical characteristic, idea, or behavioral tendency), or its absence (e.g., physical strength), that hinders but does not significantly preclude one from reaching one’s objectives. Such traits can be physical, emotional, cognitive, and/or social in kind, with vulnerability measured using a concept universally recognized as “status.” Mutual. Most types of vulnerabilities are shared by all members of our species. When an individual’s or group’s vulnerability is in some way highlighted, the prior status relationship—their standing relative to others—becomes altered. Affirmation. Laughter is a reminder that each party has shortcomings. Sometimes this statement is welcomed; however, sometimes not. Vocal. Laughter is characteristically a vocal signal, although there are several nonvocal forms of communication that can also relate a sense of shared oneness and common cause.