This guidance note, about Energy service companies (ESCO), was prepared under the Energy Efficiency Outreach activity of the World Bank’s Europe and Central Asia region. The activity is sponsored by the Energy Sector Management Assistance Program, a multi-donor technical assistance trust fund administered by the World Bank and cosponsored by thirteen official bilateral donors. ESCO can aid energy efficiency efforts by providing technical skills, assuming performance risks, facilitating access to finance from commercial lenders, and enabling energy users to repay initial costs through future savings. Although many attempts to encourage the development of ESCO markets in developing countries have failed, some recent experiences demonstrate how governments can help by promoting simple business models; facilitating ESCO financing; making legislative, regulatory, and policy changes; and creating demand. The challenges are real as the ESCO models are complex and require strong legal, financial, accounting, and business infrastructure, which is often lacking in developing countries. However, a combination of simple ESCO models, dedicated financing, enabling policy, regulatory initiatives, and increased public sector demand has resulted in the development of sizeable ESCO markets in some countries.