This Quick Insight report describes locational marginal emissions rates (LMERs) and how they are calculated, how they are used, who is interested in using them and what entities currently are providing LMERs for large power systems in the United States. It explores different approaches that may be used to calculate LMERs and the strengths and weaknesses of these approaches. This reports also summarizes EPRI’s analysis of a PJM dataset that calculates marginal emission rates at a five-minute granularity.