Wind energy has been used as far back as Roman Egypt. Like solar energy, wind energy faces an intermittent nature of its source. People commonly refer to wind and solar energy as variable renewable energy sources. The intermittency in wind presents a number of challenges to wind energy operations. The commercial competitiveness of wind energy can benefit a great deal from a good understanding of its production reliability, which is affected by the unpredictability of wind and the productivity of wind turbines. Responding to changes in wind speed, pitch control turns the blades in relation to the direction of the incoming air flow, adjusting the capability of the turbine to absorb the kinetic energy in the wind or the turbine’s efficiency in doing so. The chapter also presents an overview of the key concepts discussed in this book.