This chapter mainly introduces the basic properties and principles of metal oxides in the field of electrochemical water decomposition. Electrochemical water decomposition is a promising method to convert electric energy into ideal clean hydrogen fuel. The basic principles of metal oxides used for the electrocatalytic decomposition of water are discussed in this chapter. It mainly includes the description and definition of metal oxides in the field of electrochemistry, as well as the classification of metal oxides. Furthermore, we describe the basic properties of metal oxides and clarify the electrochemical advantages of metal oxides in the field of electrochemical water decomposition. Finally, we briefly describe metal oxidation. In this chapter, the progress of electrochemical water decomposition and its advantages and disadvantages as electrocatalysts were compared. We hope to provide a relatively comprehensive picture for the development of high cost-effective electrocatalysts through the description of relevant contents.