This book gives an overview of statistical turbulence-modelling with applications to oceanography and limnology. It discusses how these models can be derived from the Navier-Stokes equations, step by step simplifications result in models applicable to numerical simulations for realistic solutions. Results from one-dimensional simulations are shown for various oceanic and limnic water column studies. The integration of these turbulence models in three-dimensioanl models is discussed and some selected results are shown. The two-equation turbulence models prove to be a good compromise between accuracy and economy are published as a FORTRAN source code on the internet in the framework of the General Ocean Turbulence Model (GOTM) - see URL: http.//www.gotm.net. This web site also provides forcing and validation data for several idealized scenarios. The book and the home page enables graduate students and researchers to understand the theory and provides tools for the models.