The ain of this work was to provide a deeper understanding of the electronic and geometrical structures of HA when substituted by various ions considered important in the field of biomaterials. Calculations were carried out using Density Functional Theory, (DFT), on bulk and surface, substituted-HA structures. Particular attention is given to the substation of phosphate ions by silicate ions. Bulk structures are investigated with supercells and the Virtual Crystal Approximation, which simulates low concentrations (up to 2.8 wt%) of silicon in the unit cell. The amount of silicon that can be substituted into a single cell is limited by the need for charge compensation, as the silicate ion has a formal charge of -4 and phosphate -3. Charge compensation is therefore explored, showing that hydroxyl-deficient HA is more favourable than stoichiometric HA when silicon is introduced. The HA-britholite-(Y) solid state series is also investigated, using geometry optimisation and theoretical NMR spectra, as a potential way of increasing the silicon content of a unit cell by charge compensating with the replacement of a +2 calcium ion by a +3 yttrium ion. Further substitutions of titanium and magnesium are also thoroughly investigated with the single unit cell model. A HA (100) surface slab is also constructed and electronically optimised. This model is used in the study of surface structures and interactions and is compared to previous experimental and theoretical results. Substitution of silicon into the surfaces is investigated in addition to protonation of surface phosphate and silicate ions and the adsorption of a glutamic acid fragment.