To understand the evolution of the immune system and its related genes, studies on reptiles are essential because they occupy a key phylogenetic position as a sister group to both birds and mammals. Reptiles comprise diverse groups presenting vast differences in morphology, reproductive, and developmental characteristics among its clades. They form the link between anamniotic amphibians and amniotic birds and are the first terrestrial animals to encounter full-blown land pathogens. Though much is known on the innate immunity of reptiles, the cellular components and cell-mediated immunity are largely unknown. Thus, the study of cellular immunity in nonavian reptiles will fill an important gap in reconstructing the evolutionary history of amniote vertebrates, which to date has been poorly represented. This chapter aims to annotate the missing links in reptilian immunology.