The reproductive biology of poriferans is still poorly understood. We have investigated the sexual reproductive biology of seven demosponge species, six of them from the Mediterranean (Corticium candelabrum, Crambe crambe, Raspaciona aculeata, Axinella damicornis, Chondrosia reniformis, and Petrosia ficiformis), and one from the Pacific coast of Canada (Asbestopluma occidentalis). The thesis consists of a general introduction, 7 different chapters and a general discussion.Chapter 1. It is well established that in temperate regions invertebrates restrict their reproductive cycles to the warm periods. The sexual cycle of A. damicornis, C. candelabrum, C. reniformis, and R. aculeata is very different in timing and duration, despite all of them shared habitat and thermal regime. While the gametogenesis of R. aculeata and C. reniformis underwent during summer and autumn (warm periods in the Mediterranean), the gametogenesis of C. candelabrum and A. damicornis occurred during winter. Therefore, the relationship between gametogenesis and temperature in temperate waters was not straightforward, and many different relationships appeared.Chapter 2. The oogenesis of C. candelabrum resulted in a surprising long process, with continuous production of oocytes. However, the oocyte maturation extended for 7/8 months during autumn and winter. Spermatogenesis occurred during 4 or 5 at the end of the oocyte maturation (spring and summer). Spermatozoans were primitive but possessed a true C-shaped acrosome. Chapter 3. The gametogenesis of the oviparous demosponge P. ficiformis occurred during autumn and early winter. Large oocytes and round primitive spermatozoans with proacrosomal vesicles were released to the water on December. Recently fertilised eggs were placed in Petri dishes. Since no free-swimming larva was observed, the development in this demosponge was suggested to be direct.Chapter 4. The reproduction of the carnivorous sponge A. occidentalis was studied under light and electron microscopy. It was a contemporaneous hermaphroditic sponge with clusters of oocytes and very complex spermatic cysts. The fertilisation mechanism was unusual for the phylum Porifera, and shared many similarities with the feeding mechanism of this carnivorous sponge.Chapter 5. The spermatogenesis of the common Mediterranean demosponge C. crambe was investigated by light and electron microscopy. The mature spermatozoan was a extremely modified cell, with the body bent at the level of the flagellum insertion, a true acrosome, and a striated rootlet that connected the basal body to the mitochondrion. In addition, such modifications resembled to the sperm morphology of a phoronid, in a well example of adaptive convergence. Chapter 6. The oogenesis in A. damicornis and R. aculeata was very similar, except for the duration. While it extended for 5 months in R. aculeata, A. damicornis required 7 months to complete it. The differences resided in the vitellogenesis. Yolk was almost exclusively auto-synthesized in A. damicornis from digestion of bacteria. However, in R. aculeata the process shortened because of the help of nurse cells in creation of yolk. Chapter 7. Unspawned sperm and precursors of sperm cells were phagocytosed by motile phagocytic cells in the spermatic cysts of P. ficiformis and R. aculeata. All these features observed for the first time in demosponges revealed that sponges contain many complex features and are capable of complex processes that are usually regarded to higher invertebrates and vertebrates.