During the development of the gastrointestinal nervous system, neural crest cells must first migrate into and colonise the entire gut from stomach to anal end. These cells form the enteric nervous system, which gives rise to normal gut function and peristaltic contraction. Failure of the neural crest cells to invade the whole gut results in a lack of neurons in a length of the terminal intestine. This is a potentially fatal condition. We are developing mathematical models of neural crest colonisation. We are using continuum population-level and agent-based cell-level modelling approaches to replicate and connect the current population-level and cell-level experimental data. These models are being used to generate experimentally testable predictions.