This book gives the first comprehensive overview of turbulence modelling from both the conventional and statistical-theoretical viewpoints. The mathematical structures of primary turbulence models such as algebraic (turbulent-viscosity-type), second-order, and subgrid-scales ones are elucidated, and the relationship between them is shown systematically. This approach is extended to turbulent or mean-field dynamo that plays an important role in the study of the generation and sustainment mechanisms of magnetic fields in astro-geophysical and fusion phenomena. Finally, turbulence modelling is shown to be a concept possessing a wide range of applicability in both the practical and academic senses. Readers are expected to have a basic knowledge of fluid mechanics at a graduate level and beyond. The important properties of turbulence necessary for turbulence modelling, however, are explained in a self-consistent manner. This book is therefore suited for both graduate students and researchers who are interested in turbulence modelling and turbulent dynamo.