Isospora suis, an intestinal protozoan parasite of swine, is the causative agent of neonatal coccidiosis, a disease with high morbidity in affected pig-breeding units and consequently of high economic importance. Infection leads to damage of the mucosal surface in the jejunum and ileum and to non-haemorrhagic diarrhoea. As a result, weight gain of piglets is reduced and secondary infections with other enteric pathogens may lead to increased mortality. Despite its economic and veterinary importance, host-parasite interactions are still poorly understood. To examine these interactions experimental infection models are established using outbred piglets infected with defined numbers of parasites on different days of life. This review discusses the life cycle of Isospora suis and the clinical and parasitological characteristics of porcine neonatal coccidiosis including pathology, and compare the different experimental infection models and the tools for studying Isospora suis in vitro. Moreover, it summarises findings about natural age resistance of pigs against infections with Isospora suis, our current knowledge about immune response to other coccidial infections, e.g. with Eimeria spp. in different hosts, and gives a short overview on peculiarities of the porcine immune system and its development in young animals which may play a role in porcine coccidiosis.