The early work of Brattain and Bardeen demonstrated that under illumination, a semi-conductor surface develops a potential which can be measured with non-contact capacitive coupling. Since then, the surface photo-voltage (PV) effect has received sporadic experimental as well as theoretical considerations. A number of experiments have been reported (with and without contacts) which make use of both DC and AC surface PV. In the latter form, the incident light is modulated at frequencies ranging from a few Hz to a few MHz. Both the DC and AC embodiments of the surface photo-voltage effect have been used to study various minority and majority carrier transport mechanisms in the presence of various degrees of inversion (or accumulation) of the semiconductor surface. Theoretical treatment of the PV effect has also been sporadic and lacking in completeness. Existing models for the PV effect are either too simplified to accurately represent a physical semiconductor surface, or are too general to be useful for the interpretation of experimental data. It is nevertheless clear from theoretical considerations and from existing experimental results (using incident photon energies above and below the semiconductor bandgap) that the PV effect contains valuable quantitative information which is specific to the space charge region of a semiconductor surface or junction. It is the object of this paper to demonstrate that the PV effect affords the opportunity to quantitatively measure many of the technologically important properties of a semicon-ductor surface or junction without contacting the sample and with a spatial resolution that is consistent with very large scale integration (VLSI) technology. It is also shown that non-contact PV is very sensitive to junction bias and can therefore be used to detect the on-off state of an operating transistor.