Regenerative processes are very common in echinoderms. Regeneration takes place through the recruitment of progenitor cells (undifferentiated or differentiated) accumulating in the lesion region. The crucial point of the "progenitor" elements involved in regenerative processes, in terms of cell recruitment, sources and fate, is a central problem, relevant for its topical interest. Crinoids such as the feather star Antedon mediterranea possess a spectacular regenerative potential. The echinoid Paracentrotus lividus possess a large amount of coelomic fluid containing several cells, many of which contribute to repair (test regeneration) and are apparently quite similar to those already characterized in A. mediterranea. As far as these experimental models are concerned the present book provide an extensive in vitro approach which represents the first attempt to culture the cells involved in echinoderm regeneration processes already described and partly characterized in vivo. Integrate and comparative analysis are addressed to identify the different cells and their involvement in processes such as migration, proliferation, differentiation and de-differentiation.