Mechanics Today, Volume 4 focuses on solid mechanics and applied mathematics. This book is divided into six chapters. Chapter I provides a general description of the basic features and relevant concepts of mixed boundary-value problems in mechanics. The problem of crack extension in a solid under arbitrary loads is discussed in Chapter II, emphasizing the crack growth that leads from a planar to a nonplanar configuration. The third chapter reviews various methods of solving the scattering of elastic waves by inclusions. The interactions of electromagnetic field with deformable bodies in motion are elaborated in Chapter IV, while problems involving solids carrying high electric currents or being placed in high magnetic fields are deliberated in Chapter V. The last chapter concentrates on the implications of the second law of thermodynamics, and consequences of thermodynamic material stability and its corresponding restrictions on the evolutionary equations for internal variables. This publication is useful to specialists, but is also beneficial to non-experts with sufficient background in applied mechanics.