Sessile animals show peculiar reproductive strategies which are linked to their speciation mechanisms. The A. considers the adaptive role of both asexual and sexual reproduction. Asexual reproduction through budding builds colonies — coloniality being an important adaptation for substrate occupation. In the solitary sessile animals coloniality may be replaced by gregariousness. Sexual reproduction, often accompanied by hermaphroditism and sex reversal, has a basic function, besides the maintenance of an intraspecific genetic variability, in species dispersal and habitat selection through the recruiting larvae. The high degree of morphological variability, which is an adaptation to sessile condition, is reached — as in plants — not only through genetic polymorphism depending on sexual reproduction but also through phenotypic plasticity. Sessile macrofauna probably developed originally as an adaptive condition to unstable and scattered substrata, under the influence of unpredictable physico-chemical factors and followed a generalist (r-selected), opportunistic strategy. Later on, however, also in relation to the evolution of coloniality, a more selective, specialist (K-selected) strategy has been developed. This hypothesis finds support in some paleontological data. To-day the two opposite, r-selectel and K-selected, strategies are both present. In conclusion, the reasons for the evolutionary success of marine sessile animals, which still constitute the bulk of some of the more productive world ecosystems as the coral reefs, may be found not only in their extraordinary trophic plasticity but also in the large array of their sexual and asexual reproductive strategies.