This book presents two new imaging methods to measure the effective series resistance and sheet resistance on solar cells under operating conditions. The methods are contactless-and are therefore non-destructive-and offer results with high spatial resolution. A standard photoluminescence imaging setup for data acquisition is used for both methods. The high optical resolution typical of these systems allows a simultaneous measurement of millions of data points within seconds, thereby giving the presented methods the potential to be employed in time-sensitive offline process control in manufacturing. Novel to both methods presented in this work is their unprecedented robustness to the influence of other electrical and material properties on the resulting data, a weak point of many other optical imaging methods. Further, no calibration procedure is required, nor is any information regarding the electrical characteristics of the solar cell. This makes it possible to correctly measure solar cells with strong lateral variation in their material properties, such as multicrystalline solar cells.