In today’s competitive business environment, segmentation should be the foundation of any marketing strategy by utilities or energy service providers. Whether used as a tool for identifying existing profitable customers, building loyalty, acquiring customers, or developing new products, segmentation provides an intimate understanding of customer needs and wants. E Source elicited ideas, advice, and case studies from key experts, vendors, and marketing professionals with experience in implementing segmentation. This report uncovers the major lessons that utility marketers should keep in mind when considering a residential segmentation effort. (RES-10; January 2001; 43 pages)