During development of the nervous system, neurons should be appropriately positioned to enable them to make the right functional contacts. Neurons do not immediately migrate to their correct location, but instead regular arrangements gradually emerge from randomly arranged cell populations. This phenomenon has been studied often in the retina, due to its relatively simple layered organisation. In this review, I highlight the principal mechanisms that are thought to be involved, and how mathematical modelling has helped to further our understanding of the role of these processes upon mosaic formation. Three developmental mechanisms are studied in detail, namely, lateral migration, cell fate and cell death. As a case study, I then consider which mechanisms might be involved in the formation of retinal ganglion cell mosaics.