Biomineralization is the process by which mineral crystals are deposited in an organized fashion in the matrix (either cellular or extracellular) of living organisms. Over the past 25 years, new insights into the mechanisms that control these processes have been obtained, yet questions asked then still persist, especially in terms of vertebrate mineralization. Specifically, there are still debates concerning the chemical nature of the first mineral crystals formed in bone, dentin, and cementum; the factors leading to the initial deposition of these crystals; and the functions of macromolecules found associated with these crystals. In this review, emphasis is placed on the currently accepted answers to these questions, drawing insight from nonvertebrate systems. It is suggested that there are redundant calcification mechanisms and that, by taking advantage of our current knowledge of these mechanisms, opportunities will be provided for therapeutic manipulation of diseases in which biomineralization is impaired.