The performance of zinc oxide (ZnO) surge arresters has been considered in the present work, in particular, the effects of preconditioning on their protective level. The work involved development of laboratory experimental set-ups and theoretical modelling. The effect of single and double impulse stresses on ZnO surge arresters has been investigated using the constructed double impulse test facility which provides means of testing surge arresters with two successive 50kV, 5kA impulses of similar, and for the first time, opposite polarities. The time interval between both impulses can be as low as 10 ms. Tests on two types of 15kV rated ZnO surge arresters show that the application of double impulses of similar shape and amplitudes with the same or opposite polarities and 10 ms or longer time intervals has no significant effect on the protective performance of ZnO surge arresters. However, a characterisation of the ZnO material based on single impulse test data suggests that the V-I characteristic of the material may have a dynamic response resulting in departure from the static curve. A computer simulation of the single impulse test circuit is included.