The reactivity of bioglasses, which is related to the dissolution of cations and orthosilicate groups in the physiological fluid, depends on the key structural, features present at the glass Surfaces. On the basis of the 1 composition and the synthetic routes employed to make the lass surfaces with very different characteristics and thus presenting different mechanisms of dissolution can be observed. In this paper, the surface :structures, of two very different bioglass compositions, namely 45S5 (46.1 SiO2, 24.4 Na2O, 26.9 CaO, and 2.6 P2O5 mol %) and 77S (80.0 SiO2, 16.0 CaO, and 4.0 P2O5 mol %), have been investigated by means of periodic DFT calculations based on a PBE functional and localized Gaussian basis Set as encoded in the CRYSTAL. code. Our calculations show that the two glass surfaces differ by the relative amount of key structural sites such is NBOs, exposed ions, orthosilicate units, and small rings. We have demonstrated how the number: of these sites affects the surface stability and reactivity (bioactivity).