This text targets advanced undergraduate students, graduate students and practicing aquatic scientists who seek to understand effects of flow on aquatic processes but have had little prior exposure to fluid dynamics. It provides a self-contained introduction to flows at small scales within oceans and fresh waters in ubiquitous settings, such as boundary layers and dissipative vortices, wherein viscosity suppresses inertial forces. Diagrams, graphs and equations enable reader calculations of viscous flow effects. Detailed derivations include drag forces, solute fluxes and particle encounter rates. Applications described include the effects of shape and orientation on drag in steady and unsteady flows, nutrient uptake by bacteria and phytoplankton, quorum sensing, particle coagulation and suspension feeding. Teachers of biological fluid dynamics will find this book to be a rich, student-tested source of examples and applications of low Reynolds number flows. Its coverage of both bounded and unbounded flows carefully specifies the limits of low Reynolds number behaviors as flow velocities increase, and indicates the consequences when those limits are approached and exceeded.