The book Biological Feedback by René Thomas and Richard D'Ari was published in 1990 by CRC Press, Inc. In 2003 the Taylor & Francis group purchased CRC Press. With the kind authorisation of Taylor & Francis the book is now made available free of charge to the general public. The book presents a method of mathematical modelisation of biological and other systems, allowing one to extract readily from the graph of interactions the system's behaviour and its steady states.