Bergmann's rule (BR) is a classical eco-geographic rule that relates body size of homoiothermal animals with environmental temperature (or latitude). Contemporary data indicate that latitudinal clines in body size, predicted by BR, also exist in ectothermic organisms and in some groups appear to be fairly common. Despite plenty of data from literature, it is still impossible to estimate the frequency of Bergmann's clines occurrence in ectotherms as thoroughly as it is done for homoiotherms. Within large taxa of ectotherms (Arthropoda, Mollusca, Pisces), Bergmann's clines occur alongside with anti-Bergmann's and U-shaped ones. Since classic explanation of BR, which is based on thermoregulation principles, is not applicable to most ectotherms, quite a few hypotheses have been proposed that appeal to other foundations in search of such an explanation. One part of them suggests a direct modifying influence of temperature, though most authors look for adaptive sense in increase of ectotherms size at high latitudes and/or low temperatures. It appears that a single universal explanation of all the cases of Bergmann's variability in ectotherms can not be formulated. Observed clines, as it seems, arise as a result of synergetic interactions between several factors that are covaried with latitude (or altitude above sea level). It is not always possible to estimate the exact contribution of each one of them.