Chitinozoans and graptolites are the main components of preserved Ordovician zooplankton. As with much of the modern plankton, the 'first-order' species distributions of Ordovician plankton reflect water masses defined on the basis of sea surface temperatures. For 'time slices' of less than a few million years, zooplankton distribution patterns can be used to infer latitudinal sea surface temperature gradients, key palaeoceanographical boundaries and the position of Ordovician climate belts. Here, using two Late Ordovician time intervals - the early Sandbian and Hirnantian - we review how zooplankton distribution patterns identify Late Ordovician cooling and reflect the development of severe icehouse conditions.