Biology is immensely complex - the number of biological functions that could be encoded by the human genome is uncountably large; the number of different proteins is immense; the range of length scales covers at least nine orders of magnitude; and the range of timescales covers perhaps fifteen orders of magnitude. It is widely believed that we will not understand biological function without the assistance of mathematical and computational tools. Is this a reasonable belief, given the complexity? I will discuss how we can approach the problems of cellular behaviour, with examples taken from modelling the behaviour of epithelial tissues.