This book presents a method for the analysis of ecological systems using numerical models. It describes the development of these models by means of relational diagrams and rate equations. The calculation of the dynamic behaviour of the models is studied through small computer programs. The approach is illustrated by treating examples ranging from simple exponential growth to the development of an insect population and transport phenomena in soils. About a hundred worked examples clarify the theoretical concepts presented and give the book a tutorial nature. The examples often illustrate the process, from defining the problem to the development of a computer program. Examples include models about yeasts, prey-predator interactions, pollutant and organic matter dynamics and heat flow in soil. Simulation languages are used to develop the programs, so that the need to learn technical programming details is minimized and attention can be focussed on the ecological problem. The use of these languages, which may be obtained via the editor, are described in the text. The book concludes with the presentation of a simulation environment in Fortran that is in fact produced by one of the simulation languages. This Fortran environment supports even more complex modelling than the simulation languages and, moreover, assures a high model portability. Those interested in applying systems analysis and simulation to the study of ecological processes, such as biologists, agro-ecologists and soil scientists, will find this book a valuable introduction. After studying the book readers will be able to develop their own models and to critically follow the literature on more complex ecosystem models