Two methods are developed by which control systems that are non-linear in the state variables but linear in the inputs may be approximated by bilinear systems in the neighbourhood of a steady state. For both methods the approximations can be made successively more accurate at the expense of higher dimensionality, and can be a considerable improvement over the linear approximation. The resulting dimensionalities are determined as functions of the accuracy. Illustrative examples are given, and finally the two models are compared. Both are found to have considerable merit.