This chapter discusses ultrathin oxides as applied to silicon and to a lesser extent III–V semiconductors primarily in the context of solid-state photovoltaics and photoelectrochemical fuel generators. The general reasons for the use of these oxides and some important vocabulary are first introduced. A discussion of various techniques for the creation of thin oxide films follows with a focus on the often-employed atomic layer deposition technique. The chapter concludes with three different case studies. First, the use of thin oxide films for photoelectrochemical applications is discussed, where the primary objective is to prevent corrosion reactions. Second, passivation of surface defects for solid-state photovoltaics is discussed, with a focus on aluminum oxide. Third, the role of these materials in carrier-selective contacts in photovoltaics is discussed.