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Egg maturation, spermatogenesis, sex determination, and chromosomal organization are reviewed in aphids with emphasis on the complex life cycles which makes the cytogenetical mechanisms of this group very peculiar. Maturation of the parthenogenetically developing egg is still a matter of controversy in that two opinions exist: that of those who consider it a modified meiosis, and that of those who consider it an ordinary mitosis. For male production, an oocyte needs to lose half of its sex chromatin during the single egg maturation division. This process is environmentally controlled by a change in length of the daily photoperiod. In male meiosis, both pre-reductional and post-reductional divisions have been reported. At anaphase I, the single X-chromosome gets stretched on the spindle, and passes into one of the secondary spermatocytes which receives more cytoplasm and undergoes the second division, while the other degenerates. At present, over 4000 species of aphids have been reported the world over and chromosomes of about 20 of these species have been investigated, but only for a small minority have the homologous chromosomes been clearly identified.