Chapter 3 covers the natural history of reproduction. A first section on asexual reproduction deals with the different forms of cell division in unicellular prokaryotes and eukaryotes. A short interlude introduces the notion of sex and describes sexuality phenomena uncoupled from reproductive processes, both in prokaryotes and in unicellular eukaryotes. The main types of sexual reproduction (gametogamy, gamontogamy, autogamy) are described, as are the distinction between sexes and mating types and the different ways in which sexual roles can be distributed within the population. Short paragraphs are devoted to secondary sex characters and to conditions such as aneuploidy, gynandromorphism and intersexuality. Attention is then shifted to the reproductive organs of the metazoans and the morphology of eggs and spermatozoa, and also to the reproductive organs of the plants and the morphology of their gametes. Finally, the fate of gametes is described, both in typical biparental reproduction, with particular regard to its ecological context, and in uniparental sexual reproduction (self-fertilization, parthenogenesis, gynogenesis, androgenesis and hybridogenesis).