An experimental method will be described for extracting the non-linear I-V curve from intracellular-voltage traces. The method provides empirical grounds for the choice of which type of integrate-and-fire neurone to use to model different classes of cells. Experimental measurements of cortical pyramidal cells and basket-cell interneurones will be used to illustrate the method. To complement the experimental modelling, a simple theoretical framework for the calculation of firing-rate response properties of any non-linear integrate-and-fire model or recurrent network will also be described.