In this paper we determine the dynamic buckling load of an imperfect spherical shell under an impulse and also examine the effects of non-linear terms on the initial post dynamic buckling behaviour of the structure. The results show that: (a) neglecting the axisymmetric and non-axisymmetric imperfections implies neglecting, respectively, the effects of the axisymmetric and non-axisymmetric non-linear buckling mode contributions, and (b) the coupling effects of the buckling modes are felt only if none of the imperfections are zero.