Endosymbiosis is a primary force in eukaryotic cell evolution. In order to understand the molecular mechanisms involved in this mutualistic relationship, experiments to reproduce endosymbiosis are indispensable. The ciliate 'Paramecium' is an ideal host for performing such studies. Topics presented in this volume are: the origins of algal and bacterial symbionts in 'Paramecium', the diversity of endosymbiotic bacteria, such as 'Holospora' bacteria and especially 'Chlorella' species, as well as the infection and maintenance processes. The metabolic control, the regulation of circadian rhythms and photobiological aspects of the mutualistic association, as well as the killer effect of 'Paramecium' and its causative agents are further points discussed.