This is the third, penultimate volume in the Toward a Theoretical Biology series. The contributors agree that there is a major problem in finding methods of dealing with the great complexity of biological systems. Molecular biology has given us considerable insight into the nature of the elementary units and processes of life, but to understand how these are put together to form systems that are usually too complicated to be analysed completely, but exhibit global properties of simplicity, presents biologists with an intellectual challenge that physical sciences and chemistry must also face. The problem is approached from several different angles: quantum physics, topology, and statistical mechanics. A stimulating discussion is recorded: that the behaviour of randomly constructed networks exhibits simplicity. Thoughtful analyses of complexities in such basic biological processes as the genetic control of differentiation, evolution, and ecology is also included. Some of the questions dealt with are: What kinds of theories should we wish to have in connection with developmental biology? And have we got them? The subject matter of 'Organization Stability and Process' is defined as the basic concept of biology. None of the contributors herein contained is a molecular biologist in the modern sense, but molecular biology casts a shadow over this work, at least in so far as they challenge its interpretative aggressiveness and its enthusiastic but unendearing self-confidence and self-sufficiency. This volume inaugurates a new and authentic style of scientific literature. The contributions are thoughtful, imaginative, illuminating, and exceptionally well written.