Zinc oxide is a promising material for semiconductor device applications. It has a direct and wide band gap (3.37eV) and large free - excitation binding energy (60 meV) at or even above the room temperature. The following chapters are discussed in this book. Chapter 1 Introduction and an overview of metal oxide semiconductor was discussed. Importance of pure, doped and transition metal (TM) codoped ZnO nanomaterials towards the various applications such as optical, catalytic, magnetic and antimicrobial studies were also discussed. Chapter 2 Scope of the present investigation are focused. Chapter 3 Detailed experimental procedure for the synthesis of ZnO, Ru doped ZnO and Ru-Mn codoped ZnO and photodegradation, heterogeneous catalytic activity and antimicrobial activity are presented. Chapter 4 Deals with the results of the studies on the structural, optical and photodegradation of azo dyes using pure ZnO and Ru doped ZnO. Chapter 5 Describes the heterogeneous catalytic oxidation of alcohols and antibacterial studies of ZnO and Ru-Mn codoped ZnO nanostructures and their structural, optical and magnetic properties. Chapter 6: Conclusion.