In animal embryos, cell fate specification coordinates with the spatial organisation of cells into distinct domains, leading to the establishment of embryonic boundaries. Multiple hypotheses have been proposed to explain the mechanisms underlying cell sorting, focussing on differences in tissue adhesiveness, contractility, or cell motility. Here, we summarise the main mathematical models of cell sorting and review recent findings showing how these models yield new insights into the problem. Finally, we briefly comment on the limitations of each approach and the importance of identifying the key biophysical ingredients when building a model.