The population balance methodology provides a powerful framework for studying polydisperse entities such as aerosols, crystals and bubbles. This self-contained and accessible book explains how this theoretical framework can be employed across a wide range of scientific, engineering and environmental problems. The methodology is explained step-by-step, showing readers how to use these techniques by formulating the population balance problem, choosing models and implementing appropriate solution methods. Particular focus is given to the coupling of the population balance with fluid mechanics and computational fluid dynamics (CFD), in both laminar and turbulent flows. Applications of the population balance methodology are explored in case studies including nanoparticle synthesis, soot formation and crystallisation, and sample open-source code is provided. This book will be valuable to researchers across a range of disciplines including chemical and mechanical engineering, physics and environmental science, and can be used as a resource for advanced undergraduate and graduate courses.