This book surveys theoretical models in three broad areas of biology (the origin of life, the immune system, and memory in the brain), introducing mathematical and (mainly) computational methods that have been used to construct simulations. Most current books on theoretical biology fall into one of two categories: (a) books that specialize in one area of biology and treat theoretical modes in considerable depth; and (b) books that concentrate on purely mathematical modes, with computers used only to find numerical solutions to differential equations, for example.Although some mathematical modes are considered in this book, the main emphasis is on stochastic oomputer models of biological systems, and are likely to be the preferred modelling methods of the future. By considering three different areas in biology, the book shows how several of these modelling techniques have been successfully applied in diverse areas.Put simply, this book is important because it shows how the power of modern computers is allowing researchers in theoretical biology to break free of the constraints on modelling that were imposed by the traditional differential equation approach.