Although the question is relatively simple to state, it turns out to be difficult to answer when asked in relation to a meshed nationwide gas transportation network and when taking into account the technical details and discrete decisions, as well as regulations, contracts, and varying demands involved. As well as providing an introduction to gas transportation planning, this book discusses in detail several mathematical models that address gas transport within the context of its technical and regulatory framework. It also shows how to solve the models using sophisticated algorithms, and includes examples of large-scale applications of mathematical optimization to this real-world problem. Readers will find a glossary of terms, tables listing the physical and technical quantities and constants used throughout, and a reference list of gas business literature.