Nature has evolved sophisticated strategies for engineering hard tissues through the interaction of proteins, and ultimately cells, with inorganic mineral phases. The remarkable material properties of shell, bone and teeth thus result from the activities of proteins that function at the organic-inorganic interface. A better understanding of the biomolecular mechanisms used to promote or retard the formation of mineral-based structures could provide important design principles for the development of calcification inhibitors and promoters in orthopedics, cardiology, urology, and dentistry. In addition to investigating the molecular-level basis for the recognition of biomineral surfaces and the control of hard tissue growth by proteins, the development of materials using biomimetic principles has potential applications in catalysis, biosensors, electronic devices, chromatographic separations, to name only a few. Despite the high level of interest in elucidating and controlling the structure of proteins at material and biomineral interfaces, there is a decided lack of molecular-level structure information available for proteins at biomaterial interfaces in general, and in particular for mammalian proteins that directly control calcification processes in hard tissue. The most fundamental questions regarding the secondary and tertiary structures of proteins adsorbed to material surfaces, how proteins catalyze the formation of biomineral composites, or how proteins interact at biomaterial interfaces, remain unanswered, largely due to a lack of methods capable of providing high resolution structural information for proteins adsorbed to material surfaces under physiologically relevant conditions (i.e. fully hydrated). In order to develop a better understanding of the structure and interactions of proteins in biomaterials, we have begun to utilize solid-state NMR techniques to determine the molecular structure and dynamics of proteins and peptides on inorganic crystal surfaces and within biomineral composites. In this review, we will highlight recent work that is providing insight into the structure and crystal recognition mechanisms of a salivary protein model system, as well as the structure and interactions of a peptide which catalyzes the formation of biosilica composites.