This is a book for biologists about the use of models in testing biological theories. Scientists construct models to help understand complex patterns of biological form, structure or behaviour that cannot be observed directly. The neutral model is defined as the simplest explanation that requires the fewest assumptions. The eight chapters in this book, written by a distinguished group of scientists, describe the principles of modelling, and tell how to apply models to research in molecular biology and genetics, ecology and evolution, and paleontology.