Theileria are important tick-transmitted protozoan parasites that infect wild Bovidae and domestic animals throughout much of the world. Much of our understanding of Theileria sporozoite invasion of bovine cells is based on work on T. parva, the causative agent of East Coast fever in cattle throughout east, central and southern Africa. Sporozoite entry involves a defined series of sequential but separable steps that differ in important details from the invasion process in other apicomplexans such as Plasmodium and Toxoplasma. While the morphological features of invasion are fairly well documented, the detailed biology of the individual steps is only now becoming clear. This review summarizes much of this recent work on the biology of sporozoite entry. In particular, recent studies on the role of Ca2+ and cell activation processes in sporozoite entry suggest that the initial sporozoite binding event triggers the mobilization of intrasporozoite Ca2+ and the activation of both kinase and G-protein associated signalling processes in the parasite. These processes in turn regulate the invasive capacity of the sporozoite although the identity of these parasite molecules and how they contribute to the invasion process remain to be determined.