The study of rings of continuous functions, developed systematically in the classic monograph "Rings of continuous functions" by Gillman and Jerison, had its origin in the year 1937, when Stone-Cech compactification of a Tychonoff space was formally initiated. This book deals with the study of subrings of C(X). It has been characterized in this book that which subrings are isomorphic to some C(X) for some Tychonoff space X.