This book deals with the theory of macroscopic systems. Traditionally this theory has been fragmented over a number of disciplines like thermodynamics, physical transport phenomena, sometimes referred to as non-equilibrium or irreversible thermodynamics, fluid mechanics, chemical reaction engineering and heat and power engineering. This fragmentation, the different approaches followed in presenting theory, e.g. the inductive approach as opposed to the postulational approach in textbooks on thermodynamics, many alternative representations of equations and differences in notation make it cumbersome to discern a single coherent theory of macroscopic systems. The idea of this book is to present the theory of macroscopic systems as a unified theory with equations strictly developed from a single set of principles and concepts. The book is an attempt to bridge gaps between the various disciplines. It can serve as a textbook, refresher or reference book to students of an advanced level in various disciplines, to scientists and to practising engineers working in design and development. It provides rigorous equations and their possible simplifications for use in computer models for scale-up or optimisation. Topics like exergy analysis and multi component diffusion are included. The principles and concepts in the theory of macroscopic systems com prise in addition to the mole and mass balances over a system, the balance equations for the fundamental extensive properties momentum, energy and entropy as well as the phenomenological laws on asymptotic phase behaviour and molecular transport.