Competition between plants for the capture of the essential resources for plant growth (i.e. light, water and nutrients) determines the performance of both natural and agricultural ecosystems. The topic has been studied extensively by ecologists and physiologists, leading to the development of several approaches to describe the outcome of competition. This book reviews these modelling approaches and their potential for predicting yield loss as a result of crop-weed competition. The authors provide a detailed description of the model INTERCOM and of methods to parameterize and evaluate the model using experimental data. Examples are given of how the model can be used to understand the effects of weeds on crops in field situations. The book presents a number of new developments and is aimed at theoretical plant ecologists as well as agronomists and weed scientists interested in ecophysiological approaches.