John Baez has put together an extremely interesting and timely book which brings into focus a bubbling emerging subject...The volume communicates the atmosphere of excitement and enthusiasm in the field. I heartily recommend this book to everybody working in the search for the next step [in] our understanding of the physical world at the fundamental level. * General Relativity and Gravitation, Vol. 27, No. 1, 1995 * This book is, however, best treated as a reference. Because of the breadth of topics addressed and the emphasis on connections and relationships, it may be effectively used as a bridge for a researcher from one field to discover ties to another. * Class. Quantum Grav., No. 12, 1995 * ....it may be effectively used as a bridge for a researcher from one field to discover ties to another. * Class. Quantum Grav., No. 12, 1995 *