this volume presents an account of the current state of algebraic-theoretic methods as applied to linear and nonlinear multidimensional equations of mathematical and theoretical physics. equations are considered that are invariant under euclid, galilei, schrödinger, poincaré, conformal, and some other lie groups, with special emphasis being given to the construction of wide classes of exact solutions of concrete nonlinear partial differential equations, such as dalembert, liouville, monge-ampère, hamilton-jacobi, eikonal, schrödinger, navier-stokes, gas dynamics, dirac, maxwell-dirac, yang-mills, etc. ansätze for spinor, as well as scalar and vector fields are described and formulae for generating solutions via conformal transformations are found explicitly for scalar, spinor, vector, and tensor fields with arbitrary conformal degree. the classical three-body problem is considered for the group-theoretic point of view. the symmetry of integro-differential equations is also studied, and the method of finding final nonlocal transformations is described. furthermore, the concept of conditional symmetry is introduced and is used to obtain new non-lie ansätze for nonlinear heat and acoustic equations. the volume comprises an introduction, which presents a brief account of the main ideas, followed by five chapters, appendices, and a comprehensive bibliography. this book will be of interest to researchers, and graduate students in physics and mathematics interested in algebraic-theoretic methods in mathematical and theoretical physics.