Energy system planning and operational procedures are based on adequate methodologies of forecasting energy supplies and the prices of energy carriers. Utilities will need a fast, reliable, and economical planning “tool box” to evaluate the consequences of different strategies of development and their impact on the socio-economic system as a whole in supplying energy to the consumer. Each energy company must be able to perform short-, medium-, and long-term planning, and as a consequence to carry out, for example, electric load and price prediction. The necessity for system forecasting and planning becomes even more pressing when considering the rapid changes in technology, fuel availability, environmental and financial constraints, and structural transformations in the power sector. Extreme price volatility, which can be even two orders of magnitude higher than for other commodities or financial instruments, has forced producers and wholesale consumers to hedge against not only volume risk but also price movements. Price forecasts have become a fundamental factor in energy company decision-making and strategy development. As a result of the supply stack structure, load fluctuation translates into variations in energy prices. Forecasting has become increasingly important since the rise of competitive energy markets. Short- and medium-term forecasting is important for modeling prices and valuation of spot and derivative contracts for the delivery of energy. Theoretical considerations and techniques are illustrated using case studies.