The roles of EBIC, OBIC (LBIC), spatially resolved spectral response and cathodoluminescence in determining the influence of grain boundaries in the CdTe/CdS solar cell are critically reviewed. Conventional use of EBIC in the determination of junction position and L-e in thin films is assessed. Direct evidence of grain boundary passivation from front-wall (illumination geometry) OBIC and EBIC is described. The use of non-standard injection-dependent EBIC to probe near grain boundary doping, and a model of grain boundary passivation is outlined. New results of the use of cathodoluminescence microscopy to probe individual grains are presented and discussed with regard to passivation.