This book follows a precursor volume devoted to biological calci?cation, - sued by the CRC Press, Boca Raton (Florida) in 1992. Several basic aspects of the calci?cation process were analyzed in it by outstanding authors who had unquestioned competence in their respective research areas. Its main aim was that of giving readers access to a series of papers which, even though they discussed divergent aspects of biological calci?cations drawn from the study of systems as different as vertebrate skeletons and mollusks, in vitro cultures and unicellular organisms, ectopic calci?cation and urinary stones, provided elements permitting a coherent approach to a comprehensive view of the calci?cation process in biological tissues. Now, almost 15 years after the publication of that book, a great variety of new data from a wide spectrum of biological organisms and systems has enriched our knowledge of the normal and pathological mechanisms which can lead to calci?cation. Even so, this whole process is still problematic: the new knowledge, concepts and ideas have often suggested that a de?nitive solution was close at hand, but the local mechanism through which the inorganic substance is laid down in organic matrices continues to be an elusive, largely enigmatic topic.