The economics of cogeneration and district energy are described and examined, starting with fundamentals and general economic considerations such as methods for estimating TCI, performing economic evaluations and calculating revenue requirements. A detailed case study on the economics of cogeneration is provided. For the scenario considered in the case study, the fixed and TCI, as well as fuel costs, start-up costs, and working capital (WC), are explained. The cogeneration project economics are provided, and the facility investment, modified accelerated cost recovery system and product costs are described. Modern and historical applications of the case study are examined. Various economic considerations involved in integrated cogeneration-based district energy systems are considered, including economic considerations for generation and distribution as well as economic considerations for consumers. An analogy is presented between allocating wastes for cogeneration among products, and allocating economic costs among products. Optimization schemes and procedures involving economics are discussed, to provide a foundation for detailed routines and computer codes for assessing, comparing and optimizing the benefits of such systems. Various investigations of economics of cogeneration and district energy are described.