The intimate relationship between three-dimensional vortex dynamics and topology was recognized as early as 1869 by Lord Kelvin, whose discoveries in fluid dynamics led to the development of knot theory as a well-defined branch of topology. It is only in the last 25 years, however, with the parallel stimulus of the development of magnetohydrodynamics in astrophysical and geophysical contexts, that the great potential of topological techniques in fluid and plasma has been fully recognized. This volume provides a comprehensive survey of this interdisciplinary field. The relevant background in knot theory, flow kinematics, dynamo theory, and relaxation under topological constraints, is provided by the introductory chapters of Part I. These themes are developed in the subsequent papers which are grouped under the following headings: Part II: Relaxation and Minimum Energy States; Part III: Helicity, Linkage, and Flow Topology; Part IV: The Euler Equations: Extremal Properties and Finite-Time Singularities; Part V: Vortex Interactions and the Structure of Turbulence; Part VI: Chaos, Instability, and Dynamo Theory.