This talk concerns with the roles of oxygen vacancies in various metal oxide photocatalysts and electrocatalysts. A low level of oxygen vacancies in titanium oxide can induce shallow and deep sub-band gap Ti(III) states below the conduction band, which increases light absorption but not enhance photocatalytic activity. A high level of oxygen vacancies in metal oxides may generate an abundance of states near Fermi level and induce surface plasmon resonance.