A constructive systems approach to long-term energy development forecasting (for 30 to 40 years) is elaborated and the main methods and mathematical models for implementing it are proposed. They are: -- An intersector model and method for long-term forecasting of basic indices of economic growth and national economic estimation of energy development strategies; -- An iterative scheme and optimization model for elaborating possible energy development strategies and comparison of tendencies in science and technology; -- A statistical model for forecasting final energy demand based on existing dynamics of the main indices of economic growth. These models and methods are tested on preliminary information.