This book examines the electro- and magneto-mechanics of soft composite materials and structures, and focuses on magnetorheological elastomers (MREs) and dielectric elastomer composites (DECs), which are composite materials that comprise ferromagnetic and high-dielectric/conducting filler nano- and micro-particles embedded in a soft polymeric matrix. This gives rise to a coupled magneto- and electro-mechanical response at the macroscopic (order of millimeters and larger) scale when they are subjected to magneto- electro-mechanical external stimuli. While such MRE and DEC materials and devices can become unstable at some critical electro-magneto-mechanical loading, their response may be well controlled in the post-instability regime. Moreover, recent advances on the complete electro-magneto-mechanical coupling are presented. All those aforementioned features motivate the operation of these devices in this unstable region to obtain controlled pattern formation, soft robotic motion andartificial muscles, controllable band-gap acoustic and electromagnetic properties, energy harvesting as well as actively controlled stiffness (for cell-growth). The book contains four individual chapters covering work on the fundamentals (O. Lopez-Pamies) and the modeling (M. Gei) of electroactive solids, the modeling of magnetoactive solids (K. Danas), and the analysis of elastic instabilities (Y. Fu).