This supplement to the comprehensive series "Encyclopedia of Fluid Mechanics" steps back from the topical approach to fluid mechanics, and embraces the overall subject from an entirely mathematical viewpoint. Within the pure science of mathematics, the motion of particles and fluids is described and studies without the uncertainty that can accompany experimental investigations. This volume addresses the mathematical details of model formation and development, which constitutes the basis for numerical experimentation. It is intended to stimulate and report current and emerging concepts in pure research on flow dynamics.