Can renewable energy provide reliable power? Will it need extensive backup? The energy available from wind, waves, tides and the sun varies in ways that may not match variations in energy demand. Assimilating these fluctuations can affect the operation and economics of electricity networks, markets and the output of other forms of generation. Is this a significant problem, or can these new sources be integrated into the grid system without the need for extensive backup or energy storage capacity? This book examines the significance of the issue of variability of renewable electricity supplies, and presents technical and operational solutions to the problem of reconciling the differing patterns of supply and demand. Its chapters are authored by leading experts in the field, who aim to explain and quantify the impacts of variability in renewable energy, and in doing so, dispel many of the myths and misunderstandings surrounding the topic.