The lymphoid systems of sheep and cattle contain a large number of γδ T cells, in striking contrast to the lymphoid systems of humans and mice. In neonatal animals particularly, these cells comprise the predominant fraction of T cells in the blood. Here Wayne Hein and Charles Mackay discuss what is currently known about the ontogeny, phenotype, tissue distribution and function of γδ T cells in ruminants. There are a number of interesting molecular features that characterize ruminant γδ T cells, but these do not entirely explain the high frequency of use of the γδ T-cell receptor in these animals. Studies on sheep, cattle or other animals that preferentially use γδ T cells should provide insights into the biological significance of the existence of two distinct forms of the T-cell receptor.