The one feature that distinguishes mineralized biological from synthetic materials is the presence in mineralized biological materials of a complex assemblage of macromolecules. This assemblage usually acts as the host to the guest mineral particles, a well known phenomenon in composite materials. The macromolecules can, surprisingly, also be the guests inside a host crystal, to form a sort of reverse composite material. Their presence both between and within crystals creates diverse interfaces and interphases, and these have marked effects on the mechanical properties of the material. Such “novelties” are the products of the evolutionary process, which by trial and error over long periods of time arrives at a solution that works. This may not be the most elegant solution or the most economical solution, and is therefore often unexpected. It is thus the possibility of discovering the unexpected that makes the investigation of biological materials so exciting. In some cases the biological solution to a problem may also be useful for improving synthetic materials, an added benefit. In this context, we examine here several interfaces and interphases in mineralized biological materials.