Do real stem cells and stem cell lineages exist in lower organisms? Can stem cells from one organism parasitize the soma and/or the germ line of conspecifics? Can differentiated cells in marine organisms be re-programmed to regenerate tissues, organs and appendages through novel de-differentiation, transdifferentiation, or re-differentiation processes, leading to virtually all three germ layers, including the germline? The positive answers to above questions open a new avenue in stem cell research: the biology of stem cells in marine organisms. It is therefore unfortunate that while the literature on stem cell from terrestrial organisms is rich and expanding at an exponential rate, investigations on marine organisms’ stem cells are very limited and scarce. By presenting theoretical chapters, overview essays and specific research results, this book summarises the knowledge and the hypotheses on stem cells in marine organisms through major phyla and specific model organisms. The study on stem cells from marine invertebrates may shed lights on mechanisms promoting immunity, developmental biology, regeneration and budding processes in marine invertebrates, body maintenance, aging and senescence. It aims in encouraging a larger scientific community to follow and study the novel phenomena of stem cells behaviours as depicted from the few currently studied marine invertebrates.