Policy simulation models are useful as decision-support tools when weighing up implementation of different policies. It is therefore vital that these models are tailored to the needs of policy-makers. We report on a policy modelling project – Modelling the Early Life Course – which uses dynamic micro-simulation to simulate the results of policy scenarios relating to child development. From the beginning of this project and throughout its development we have actively involved the likely ‘end-users’ of the modelling: policy analysts from four New Zealand government ministries (health, education, social development and justice). We will describe and illustrate how these end users have shaped: (i) the goals of the modelling project, (ii) the scope of the modelling project; (ii) the selection of data and statistical rules that drive the modelling; (iv) the outputs of the modelling; (v) the usability and features of the modelling tool; and (vi) the tailoring of the modelling tool to the needs of policy makers. We conclude by highlighting the many advantages of working in close partnership with policy experts to develop a policy simulation tool.