Calcium phosphate (Ca-P) formation and bone mineral structures have been the focus of research for several decades because achievements in these areas could provide valuable insights into the understanding of biomineralization. This monograph reports a systematical study on Ca-P formation, octacalcium phosphate (OCP) to hydroxyapatite (HA) transformation and bone mineral nanostructures with transmission electron microscope (TEM) techniques, including selected area electron diffraction, high- resolution TEM, fast Fourier transformation and image simulation. The study emphasizes several fundamental issues related to bioactivity and bone formation. More specifically, they include: 1) Comparative study of Ca-P formation on bioceramics; 2) In situ TEM examinations of OCP to HA transformation; 3) Ultrastructure study of HA precipitation on bioceramics in vivo; 4) Study of crystal structures in bone minerals. The target audience of this book is postgraduate students and scientists in biomaterials, biomineralization, tissue engineering and related fields. It also serves as a reference book for researchers in medicine.