Ever since meiosis was recognized as a process there has been a continuing interest in its temporal aspects. Two main types of meiotic timing experiments have been conducted: first, experiments to estimate the duration of meiosis (and sometimes its stages); second, experiments to locate the sensitive stage(s) when exposure of meiocytes to various treatments can affect meiotic chromosome behaviour (e.g. pairing or recombination). Such experiments have played an important role in increasing our under-standing of the meiotic process. The duration of meiosis has been estimated in about 70 organisms, including two prokaryotes (yeast and Chlamydomonas) and the following eukaryotes: 1 Basidiomycete (Coprinus lagopus), 2 Gymnosperms (Larix decidua and Thuja plicata gracilis), at least 39 angiosperms, and at least 26 animal species. The duration of female meiosis has been estimated in far fewer species than male meiosis. However, estimates of the duration of female meiosis are available for 6 angiosperms, Drosophila melanogaster, Xenopus laevis, and several mammals. Comparison of these data shows that the duration of meiosis is one of the most variable aspects of the meiotic process, ranging from less than 6 h in yeast to more than 40 years in the human female. Developmental holds at different stages of meiosis are common in plants and animals, and inevitably prolong the meiotic division. However, even among species without developmental holds, the duration of meiosis is very variable. For instance, in animals it ranges from about 1-2 days in male Drosophila melanogaster to more than 24 days in male Homo sapiens and several Orthopterans. Despite the large variation in the duration of meiosis three generalizations can be made: (i) first prophase is always very long compared with the remaining meiotic stages, (ii) the rate of meiotic development is very slow compared with the rate of development in dividing somatic meristem cells of the same organisms under the same conditions, (iii) the duration of meiosis is characteristic of the genotype and species. Four main factors have been recognized which effect or determine the duration of meiosis, namely (1) environmental factors (e.g. temperature); (2) nuclear DNA content; (3) ploidy level of the organism; and, (4) the genotype. Because nuclear DNA content plays a major role in determining the duration of meiosis, it has been suggested that DNA influences the rate of meiotic development in two ways: first through its informational content (the genotype), and second indirectly by the physical and mechanical effects of its mass independently of its informational content (i.e. the nucleotype). Thus, the observed duration of meiosis is the result of a complex genotype-nucleotype-environment interaction. With the obvious exception of variation caused by developmental holds, changes in the duration of meiosis usually involve proportional changes in the durations of all its stages. This is true irrespective of whether the variation in meiotic time is associated with changes in temperature, nuclear DNA amount, ploidy level, or sex difference. While results for animal species show some evidence of a similar phenomenon, the relative proportions of meiosis taken by individual meiotic stages is clearly much more variable between animal species than between plants. The duration of meiosis often has much wider implications for the organism than those affecting the meiotic process per se. Examples are given from which it is concluded, first, that the duration of meiosis can limit the type of life cycle which a species can display; and second, that in many species the duration of the meiosis is an essential adaptive feature suited to its life cycle type in its normal environment.