This book is devoted to theoretical physics methods in the theory of excitable media. The analogy between biological excitable media and active media is well known in physics and this information gives impetus to the approach used in this book. This book is the result of investigations on excitable media theory carried out by the Institute of Electrochemistry during the last decade in the USSR. The entire theory is not complete but the fundamental investigator in this field will be rewarded with insights into the practical importance of the problem and the intrinsic beauty of the theory which may be seen in the application of the theory to specific problems.