Estimates of the costs of children are used in the design of a wide range of economic and social policies. This paper provides a review of the different approaches that have been used to define the costs of a child and the estimation methods used. The paper summarises the results of Australian estimates of the costs of children since 1985. An important conclusion is that there is no unambiguous 'true cost' of a child and that the estimated costs are sensitive to the estimation method used. One way of producing costs of children for policy purposes is to take the average of all available credible estimates. This approach is used in this paper.