This book focuses on the mechanical response in viscoelastic media under isothermal and nonisothermal conditions. The viscoelastic response covered in this book is observed in a wide variety of common materials: polymers and plastics, metals and alloys at elevated temperatures, concrete, soils, road construction and building materials, biological tissues, and foodstuffs. Emphasizing the mechanical behavior of solid polymers subjected to physical aging, the book analyzes constitutive equations in thermoviscoelasticity and compares the results of numerical simulation with experimental data. After covering linear viscoelastic media at small strains, a clear approach to nonlinear constitutive equations in viscoelasticity at small strains and at finite strains is developed. The book concludes with coverage of constitutive relations in thermoviscoelasticity which account for thermally-induced changes both in elastic moduli and relaxation spectra. Written for specialists in mechanical and chemical engineering in the fields of manufacturing polymer and polymer-composite articles, this book will also appeal to specialists in applied and industrial mathematics, mechanics of continua and polymer physics who study the response of solid polymers to thermomechanical stimuli.