Very shortly after the disappearance of the non‐avian dinosaurs, the first mammals that had features similar to those of primates started appearing. These first primitive forms went on to spawn a rich diversity of plesiadapiforms, often referred to as archaic primates. Like many living primates, plesiadapiforms were small arboreal animals that generally ate fruit, insects, and, occasionally, leaves. However, this group lacked several diagnostic features of euprimates. They also had extraordinarily diverse specializations, represented in eleven families and more than 140 species, which, in some cases, were like nothing seen since in the primate order. Plesiadapiforms are known from all three Northern continents, with representatives that persisted until at least 37 million years ago. In this article we provide a summary of the incredible diversity of plesiadapiform morphology and adaptations, reviewing our knowledge of all eleven families. We also discuss the challenges that remain in our understanding of their ecology and evolution.