Molluscs (snails, slugs, clams, squid, chitons, etc.) are renowned for their highly complex and robust shells. Shell formation involves the controlled deposition of calcium carbonate within a framework of macromolecules that are secreted by the outer epithelium of a specialized organ called the mantle. Molluscan shells display remarkable morphological diversity, structure, and ornamentation; however, the physiological mechanisms underlying the evolution and formation of the shell are just beginning to be understood. Examination of genes expressed in the mantle and proteins incorporated into the shell suggests that the genetic program underlying shell fabrication is rapidly evolving. This includes lineage-specific integration of conserved, ancient gene families into the mantle gene regulatory network and the evolution of genes encoding proteins with novel repetitive motifs and domain combinations, which results in the expression of markedly different shell matrix protein repertoires in even closely-related molluscs. Here, we review the molecular physiology of shell formation with emphasis on the protein components that are particularly rapidly evolving. Nonprotein components such as chitin, other polysaccharides, and lipids are also reviewed. The high degree of novelty in molluscan biomineralized structures is discussed with emphasis on topics of recent interest including the image-forming aragonitic eye lenses of chiton shells and shell pigments. Finally, unanswered questions including some dealing with basic concepts such as the homology of the nacreous shell layers of gastropods and bivalves are discussed.