ABSTRACT. Topological conjugacy and various concepts of structural stability are defined, motivated, and criticized. Two basic problems emerge: characterization of structural stability and classification up to topological conjugacy. Solutions to these problems are outlined for linear automorphisms and the general characterization problem is discussed.This paper is an expanded version of an invited address given at the summer meeting of the American Mathematical Society in September, 1971. Much of the material here was also covered in a series of lectures given at the Institut des Hautes Etudes Scientifiques in the fall of 1971.Our notation is that of AMS 1970 subject classifications. Primary 58F00; Secondary 58F10, 58F15. Key words and phrases. Cascade, flow, topological conjugacy, differentiable conjugacy, structural stability, north pole-south pole map, hyperbolic toral automorphism, strong structural stability, absolute structural stability, relative structural stability, symplectic manifold, twist stability, topological stability, semistability, Yin-Yang problem, hyperbolic linear automorphism, in-set, out-set, on-set, Anosov diffeomorphism, selector, conjugacy selector, adjoint representation, ergodic, infinitesimally ergodic, Sobolev space, hyperbolic invariant set, nonwandering set, Q-hyperbolic, P-hyperbolic, axiom A, weak axiom A, essential spectrum, strong transversahty condition, weak transversality condition, in-set, stable manifold, out-set, unstable manifold.