The centriole is a complex cylindrical assembly found in the cells of ciliated eukaryotes. It serves two important roles in the cell: templating the growth of cilia, and forming the basis of the centrosome, which is the major microtubule organising centre in the cell. Cilia and centrosomes are involved in many cellular processes, from signalling to cell division and differentiation. As such, defects in centriole assembly can have downstream consequences on these processes and are linked to a variety of human diseases including cancer and microcephaly. The complex superstructure of the centriole has fascinated biologists for decades. It comprises a nine-fold, radially symmetric array of microtubule triplet blades attached to a central cartwheel structure. During the last two decades, proteomic analyses have identified many proteins that are associated with the centriole. However, genetic studies have shown that only a surprisingly small number of these proteins are essential for the biogenesis of the centriole. In Drosophila melanogaster, three such essential proteins, SAS-6, Ana2 and SAS-4 are required in the early stages of centriole biogenesis. In this thesis I have investigated the assembly steps involving these key players from a structural perspective. I have identified and recombinantly expressed functional domains of these proteins in order to characterise them in vitro. Using X-ray crystallography and other biophysical techniques, I have been able to define mechanisms for several steps involved in the assembly of these proteins. In collaboration with colleagues in the laboratory I have been able to investigate the biological significance of these essential assembly steps in vivo. This information has provided novel insights into the molecular, and even atomic, detail of the initial steps of centriole assembly, including an explanation of a natural point mutation involved in human microcephaly.