This is a self-contained, modern treatment of the algebraic theory of machines. Dr Holcombe examines various applications of the idea of a machine in biology, biochemistry and computer science and gives also a rigorous treatment of the way in which these machines can be decomposed and simulated by simpler ones. This treatment is based on fundamental ideas from modern algebra. Motivation for many of the newer results is provided by way of applications so this account should be accessible and valuable for those studying applied algebra or theoretical computer science at advanced undergraduate or beginning postgraduate level, as well as for those undertaking research in those areas.