Non-Intrusive Load Monitoring (NILM) describes the process of disaggregating multiple energy loads on a single circuit using signal processing techniques. The granular output of this analysis (much like an itemized phone bill) can help rate payers identify inefficiencies in their buildings and promote cost-effective energy efficiency actions. The concepts behind NIALM were originally developed in the late 1980s by George Hart of MIT and commercialized by Enetics Inc, but hardware expense, accuracy, and detection limit issues have hampered its large scale adoption. This presentation discusses the several steps of the original NILM algorithm along with recent advances and results from a field study conducted in the Fall of 2009. Examples of challenging appliances are presented that require novel and robust algorithms to isolate their power from the sum of all other appliances on the circuit.