Multiphase flow is an area of fluid dynamics that describes interactions between two or more phases of matter and is relevant across a wide range of industrial processes and natural environmental systems, from the transport of natural resources to volcanic ash flow. This book covers the topic in detail, providing clear explanations of the underlying physics behind the complex behaviour of solid particles in fluids. The forces involved in particle-fluid interactions are first used to describe the interactions between the particles, and the fundamentals of contact mechanics are then outlined and applied to model interparticle collisions. The book is illustrated with frequent worked examples and algorithms, enabling the reader to develop the required tools for simulating the flow of fluids with solid particles. This self-contained text will appeal to physicists, applied mathematicians and mechanical engineers working in this important area of research.