This report analyzes ten categories of utility-sector energy efficiency programs that have achieved high participation among targeted customer markets. Despite issues with the nature and availability of participation data, the study draws on published data sources and interviews with program contacts and industry experts to identify many examples of programs that have achieved high participation. Programs serving mass markets, such as residential customers shopping for lighting products, can move millions of products annually and penetrate 40% of the market. Some programs providing custom incentives to the largest commercial and industrial customers have achieved 50-70% participation over several years. Leading new commercial construction programs may reach 50-60% of all new projects. The report examines and analyses key factors that contribute to high participation within each of the ten selected program categories. The research reveals that an ever-growing number of utility customers are already participating in energy efficiency programs, and many more could participate.