To look deeply at play, and to place it in evolutionary, biological, cultural and contemporary context is to partially answer the question, what, really does it mean to be fully human? Or, to state it another way, if play is lost or missing, in a complex changing and demanding world, are there serious negative consequences individually and culturally that affect all who miss out on it? The eminent play scholar, Joe Frost, in his compelling book, “A History of Childhood Play and Play Environments” (1) tellingly shows that the diminution, modification, and/or disappearance of play during the latter twentieth and beginning of the twenty first century is presenting a crisis threatening our societal overall welfare, likely to last for many generations. His approach to understanding play and play deprivation has enlisted very broad and deep historic reviews of childhood play, and are in contrast to my ways of viewing play, which reflect evolutionary, biological and individual life-history clinical-experiential foci on play behavior, with a database from personal interviews built through the retrospective examination of one life at a time. My conclusions therefore have been crafted through these perspectives; nonetheless, the parallels in our differing examinations of play mirror the serious consequences from play deprivation, and each approach warrants inclusion in this play science encyclopedia.