We review recent advances in ab initio calculations of surface optical second harmonic (SH) responses of several systems: Si(001)-adsorbate surfaces, Si(111) surface, GaN(0001) surface, and Ni/Cu(001) surface. Optical second harmonic generation (SHG) has recently become a tool for studying surface electronic states. However, the interpretation of experimental SHG data has been rather difficult due to the lack of guiding theoretical predictions. For well understanding of the results of the measured SH intensity, a number of ab initio theoretical calculations have been competitively developed in these several years, and they are reviewed in this contribution. The results indicate that for simple covalent systems such as the Si surface, quantitative agreement between experiment and calculation has been obtained. [DOI: 10.1380/ejssnt.2003.57]