In this edition, we made several changes in both content and organization. In order to reduce cross-referencing, we combined the chapters on elastic equations of state and force as a function of deformation into a single chapter, Chapter 7, under the heading “Elastic equations of state and force–deformation relations”. We updated Chapter 10, “Force as a function of structure”, under the new heading, “Model elastomers”. Since volume phase transitions are now gaining fundamental importance, we moved Chapter 17, “Osmotic compressibility, critical phenomena, and gel collapse” to Part I, under the new name: Chapter 8, “Swelling of networks and volume phase transitions”. We joined Chapters 14 and 15 on “Birefringence” and “Segmental orientation” into a single new chapter, Chapter 14, “Birefringence and segmental orientation”, because both of these chapters are closely related and this merge reduces redundancy in the treatment of the subject. We significantly condensed and moved Chapter 16, “Rotational isomerization”, to the end of the new Chapter 12, “Strain-induced crystallization and ultimate properties”. This change was made due to lack of computational work and interest in the field of rotational isomerization in stretched elastomers since the first edition. We feel, however, that the importance of computational work on highly stretched chains should not be underestimated. The new chapter, Chapter 3, “The single molecule: theory and experiment”, shows new possibilities in this area. We added a new chapter, Chapter 16, “Liquid-crystalline elastomers”, due to the importance gained by this subject in recent years. The book is now reduced to 19 chapters from the original 21 chapters.