Although giant magnetoresistance (GMR) is widely used in magnetic recording, the relative importance of electronic scattering locations which contribute to--or limit--the effect is not generally agreed upon. The technologically important case of the Co/Cu/Co "spin-valve" trilayer is particularly rich since questions of surface scattering and possible channeling effects in the current-in-plane geometry (CIP-GMR) may be considered along with older questions of bulk vs. interface spin-dependent scattering. The role of surface scattering is crucial since it is widely believed that its reduction, through specularity enhancement, could provide a three-to tenfold enhancement of the spin-valve GMR. We have developed the in-situ magnetoresistance measurement as a method to locate scattering centers in the spin valve and to provide additional information on their nature. Film conductance and GMR are measured in-situ, in UHV, and in real time during magnetron sputtering, allowing the effects of interfaces and surfaces on scattering to be identified as they are created. Results from two sets of experiments will be presented. First, the onset of GMR has been investigated in NiO/Co(30)/Cu(30 Å)/Co(t) spin valves with ultrathin free layers. A main contribution to GMR is attributable to the interface alone, with a conductance response suggestive of the formation of a channeling state. Second, the response of GMR to surface treatment has been measured during coverage of NiO/Co(30)/Cu(30 Å)/Co(t) spin valves with noble metals and nano-oxide layers ("NOL"). The results in these cases do not match well with simple models of surface diffuse scattering or its reduction, and may be incompatible with them.