It is appropriate to end this book with a chapter on electric power systems. This is so because electricity is a secondary energy form, one that is usually derived from primary energy sources such as coal and oil. Thus, comparisons among the different primary fuels and their fuel cycles are crucial to intelligent planning in the power sector because electricity can potentially be made from any of them. Consequently, most of the resource, engineering, and environmental issues discussed in earlier chapters are directly relevant to choices about electric power systems. Because demand for electricity tends to grow more rapidly than that for energy as a whole during economic development, an understanding of the factors that affect electricity demand and its control is also critical for countries at all stages of development. Since the needs for financial capital in the power sector are huge and its share of primary fuels is over one-third in most countries, the economic and financial implications of the power sector exceed those for any other energy sector.