This book shows the fundamentals, theories, and applications of dielectric elastomers (DEs). This book consists of ten chapters. This book firstly introduces the definition and fundamental principle of DEs and presents typical DE materials. Then, typical DEs and their nanocomposites are described in detail. Subsequently, the fundamental of dielectric elastomer theory is also reviewed, which includes the constitutive theory, failure models, stability, transition theory, and so on. Moreover, the mechanical behavior of nonlinear dielectric elastomers, the electromechanical phase transition, electromechanical instability, dynamic mechanical behavior, and electric field induced changes are also presented. Finally, due to the great potential in intelligent bionics, aerospace, mechanical engineering, the applications of DEs are reviewed in this book, which includes the actuators with different structures, sensors, energy harvesters, robots, artificial muscles, minimum energy structures, Braille tactual displays, vibrators, loudspeaker, micropump, tunable lens, etc. This book is interesting and useful to a wide readership in the various fields of mechanics and materials science.