This chapter introduces readers to the diverse nature of the public utilities sector in the United States. After defining the electric power, natural gas, water and wastewater, solid waste and public transportation sectors, the chapter explains the justification for utilities’ natural monopolies designation. The chapter then moves to a history of the industry leading to the factors that shape the industry in the twenty-first century. For public utilities in general, many, but not all, of the problems they faced in the last several decades of the twentieth century have been solved. However, new challenges to maintaining sustainability have arisen to replace those that have been addressed.