The considerable increased interest in solar energy, and its conversion to a form of practical use has increased the importance of photovoltaic and photoelectrochemical systems. The analysis of the performance of the photoelectrochemical cells is based on the separation between the characteristics of the solid state photovoltaic junction and the electrochemical properties of the electrode/electrolyte interface. Such separation enables understanding the mechanism of operation and the effect of each part in the cell. The basic principles of solar energy conversion by photovoltaic and photoelectrochemical systems and the improvement of the performance of these systems will be reviewed. Metal oxide films like SnO₂ and TiO₂ represent important components of photovoltaic and photoelectrochemical systems since they serve as protective layers beside their function in the formation of the heterojunction with many semiconductors of practical use like silicon and gallium arsenide. The heterojunction n-Si/oxide represent the main part of stable and effective solar energy. The photoelectrochemical system n-Si/oxide/electrolyte is stable since the photovoltaic junction (n-Si/oxide) is separated from the site of the electrochemical process by the stable oxide film which protect it from photocorrosion. Incorporation of some foreign atoms in the oxide film matrix was found to improve the electrochemical performance of the system. The optical properties of the oxide films are not much affected by foreign atoms up to a concentration of 1%. The conductivity and band-gap energy of the oxide films are sensitive to foreign atom incorporation. The various mechanisms involved in both the photovoltaic and photoelectrochemical cells, their conversion efficiencies, the stability of the components of the system and the effect of foreign atom incorporation in the oxide film are discussed in terms of energy schemes. Conclusions were made concerning the use of such systems for practical applications.