This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book provides a general knowledge base and guidance for further reading of tunnels. It includes all types of tunnels, not just the larger-scale metro, road and rail tunnels, but also utility tunnels for water, sewerage and cables. The book aims to provide a comprehensive introduction to tunnel construction. It discusses the undergraduate and postgraduate students with little or no previous experience and knowledge of tunnel construction, as well as recently graduated engineers who find themselves working in this exciting field of civil engineering. The book includes the broad description of these 'categories' adopted by the British Tunnelling Society and the Institution of Civil Engineers in the United Kingdom in their tunnel lining design guide, which suggests that all types of soil and weak rocks would normally fall into the category of 'soft ground'.