There are many books devoted to ordinary differential equations con­ taining small parameters (small perturbations). The investigation of the dependence of solutions, in a finite time interval, on regular perturbations (the small parameter regularly appears on the right-hand sides of the equa­ tions) was carried out by Poincare and was practically completed long ago. However, problems connected with singular perturbations still attract the attention of mathematicians. This is what we understand by a singularly perturbed system: a system of differential equations dependent on a small parameter is said to be singularly perturbed if, as the parameter tends to zero, Cauchy's resolvent operator for the main range of time values and initial conditions from bounded sets (or the Poincare operator) converges, in a suitable topology, to a limit object acting in a space of smaller dimension. In different cases this general idea of a singularly perturbed system becomes specific and leads to numerous important and interesting problems. A certain class of these problems was only recently considered in mono­ graphic literature. This class includes problems connected with the so-called relaxation oscillations, a phenomenon well known to physicists, mechani­ cians, chemists, and ecologists. Van der Pol, Andronov, Haag, Dorodnitsyn, Stoker, Zheleztsov and others were the first to study relaxation oscillations. A comprehensive study of this phenomenon is hindered by considerable mathematical difficulties and requires the development of new asymptotic methods in the theory of differential equations. These methods, interesting in themselves, lead to the statement of new mathematical problems.