Rubber elasticity is an important sub-field of polymer science. This book is a sequel to the author's previous, more introductory book, Rubberlike Elasticity: A Molecular Primer (Wiley-Interscience, 1988). A more advanced text than its predecessor, subject areas are covered in greater depth and in a more sophisticated style, with an emphasis on molecular concepts and theories. It is expected that this comprehensive up-to-date text will replace the now classic book by L.R.G. Reloar, The Physics of Rubber Elasticity (Oxford, 1975) as a benchmark in the discipline.