This is a graduate text on turbulent flows, an important topic in fluid dynamics. It is up to date, comprehensive, and designed for teaching, it is based on a course taught by the author at Cornell University for a number of years. The book consists of two parts followed by a number of appendixes. Part 1 provides a general introduction to turbulent flows, how they behave, how they can be described quantitatively, and the fundamental physical processes involved. The topics covered include: the Navier-Stokes equations; the statistical representation of turbulent fields; mean-flow equations; the behaviors of simple free shear and wall-bounded flows; the energy cascade; turbulence spectra; and the Kolmogorov hypotheses. Part II is concerned with various approaches for modelling or simulating turbulent flows. The approaches described are: direct numerical simulation (DNS); turbulent viscosity models (e.g., the k-( model); Reynolds-stress models; probability-density-function (PDF) methods; and large-eddy simulation (LES). There are numerous appendixes in which the necessary mathematical techniques are presented. This book is primarily intended as a graduate-level text in turbulent flows for engineering students, but it may also be valuable to students in applied mathematics, physics, oceanography, and atmospheric sciences, as well as researchers and practicing engineers.