This book introduces us to the little known group of sipunculans or peanut worms, focusing on the Mediterranean species. The authors are sipunculan specialists, well acquainted with the biology, ecology and systematics of these organisms. The guide has been prepared with scientific rigor and contains updated information on the topic. It contains valuable information on the ecology and biology of the 37 species and subspecies of sipunculans present in the Mediterranean Sea, as well as an identification key, species info, distribution maps and numerous pictures of their external and internal anatomy, delving into those characteristics and structures useful for their correct identification. The illustrations and information here included constitute a helpful tool for marine biology students, naturalists and marine taxonomy specialists.