In this thesis, we give a survey of generalizations of right-perfect, semiperfect and semiregular rings by considering the class of all singular R-modules in place of the class of all R-modules. For a ring R and a right R-module M, a submodule N of M is said to be δ-small inM if, whenever N +X = M withM/X singular, we have X = M. If there exists an epimorphism p : P → M such that P is projective and Ker(p) is δ-small in P, then we say that P is a projective δ-cover of M. A ring R is called δ-perfect (respectively, δ-semiperfect) if every R-module (respectively, simple R-module) has a projective δ-cover. In this thesis, various properties and characterizations of δ-perfect and δ-semiperfect rings are stated.