Centrosomes are the major microtubule organising centres in animal cells and centrosome dysfunction has been linked to several human diseases including cancer, microcephaly and primordial dwarfism. It is unclear if there is a common theme linking these pathologies, but our research in flies suggests that centrosomes play a particularly important part in organizing the asymmetric division of certain types of stem/progenitor cells. Centrosomes comprise a pair of centrioles surrounded by pericentriolar material (PCM), and centriole and centrosome numbers are strictly controlled. Centrosomes contain several hundred proteins, including many cell cycle regulators and checkpoint proteins, but it is unclear how all of these proteins are organized to form a functional centrosome, and how cells always manage to assemble two centrosomes of the same size during mitosis. We have been studying the process of centriole and centrosome assembly using Drosophila as a model system. We find that a surprisingly small number of proteins are essential for centriole and centrosome assembly, and we are beginning to understand how these molecules work together to ensure that centrioles and centrosomes are only assembled at the right place and at the right time.