This book describes characteristic features of cryogenic systems involving two-phase flow, creates mathematical models of these systems, and then shows how the models may be used to develop optimal designs for practical cryogenic systems. The authors draw examples in the book from cryogenic fluid transport, gasification, and the stabilization of superconducting magnets. Much of this work is related to the development of large Russian systems in the areas of space technology, energy research, and particle physics.