The present study describes the early stages of biological crystals growth as observed by High Resolution Electron Microscopy (HREM). We have focused our attention on the characterisation of the initial mineralization events in human enamel and in chicken bone. In both tissues nanometer-sized particles were observed in amorphous areas. In the case of bone, these particles have a structure closely related to hydroxyapatite (HA) and a specific orientation. In more advanced situations, the nanoparticles appeared to grow in two dimensions and to form crystals with plate-like habits. In a final growth step, these crystals seemed to fuse by their (100) faces. These experimental observations allowed us to propose a four-step model for the development and growth of biological HA crystals. The two initial stages are the ionic adsorption onto an organic substrate followed by the nucleation of nanometer sized particles. The two following steps, i.e. a two dimensional growth of the nanoparticles leading to the formation of needle-like crystals and the lateral fusion of these crystals, seem to be controlled solely by spatial constraints inside the extracellular organic matrix.