This book discusses the properties of quantized vortex lines in superfluid helium-4 in the light of research on vortices in modern fluid mechanics, and gives the first comprehensive treatment of the problem. The author begins with a review of the knowledge of classical fluid dynamics which is relevant to the main topics of the book. This is followed by a presentation of basic material on helium II and quantized vortices. The following chapters deal with various different aspects of the subject, and the book concludes with substantial introductions to two currently active topics, namely superfluid turbulence and the nucleation of quantized vortices. Both quantum tunnelling and thermal activation nucleation processes are also discussed, including the Kosterlitz-Thouless transition in thin films. The author's comprehensive approach will make this book invaluable for students taking advanced undergraduate or graduate courses, and for all those involved in research on classical and quantum vortices.