This book presents in a clear and systematic manner the general theory of normal families, quasi-normal families and Qm-normal families of meromorphic functions, and various applications. Much of this book contains results of the author's research, among them is the notion of Qm-normality which includes the classical notions of normality and quasi-normality introduced by Montel as particular cases. In this book, the notion of closed families of meromorphic functions is also introduced. In addition, applications concerning the existence of the solution of various extremal problems for certain classes of univalent or multivalent functions can also be found.