This presentation will outline the theory of cross-well EM system, survey design and modelling, data acquisition configuration and workflow. Two field examples will be presented to demonstrate the usefulness of the method for fluid monitoring, reservoir characterization and locate by-passed hydrocarbon. The first example is time lapse survey in the Middle East to monitor water flood. The project consists of five surveys in two and half years. The data and interpretations clearly demonstrate the water flood pattern and effectiveness of using cross-well EM for monitoring. The second example is from a cross-well EM survey on two horizontal wells, the first ever in the world. The purpose of the survey is to understand the water breakthrough from a peripheral injector to a producer and map water flood path through a system of fractures. The data interpretation requires 3D modelling/inversion, a very challenge task considering the limited data coverage. The final resistivity model is used to compute saturation map and define by-passed hydrocarbon which resulted in placement of new wells.