This open-access textbook is an excellent introduction to systems biology, which has developed rapidly in recent years. It discusses the processes in living organisms in an integrated way, enabling the reader to understand the fundamental principles and cause-effect relationships in biology and biochemistry. The authors have chosen an original but at the same time clear way of presenting the topics, repeatedly drawing comparisons and models from the macroscopic world and making the reader aware of the unity of the laws of physics, chemistry and biology.