This chapter provides an overview of the physical principles and the application of zinc oxide in thin film silicon solar cells. Focus will be on the method of magnetron sputtering followed by a wet-chemical etching step to achieve the required surface roughness of the films. We will start with some basics of thin film silicon solar cell design and operation, including the specific requirements that have to be met by the applied TCO front contact. Most emphasis will be given to the optical and light scattering properties. This is followed by a detailed discussion of the etching behavior of sputter deposited polycrystalline ZnO:Al films. Finally, we will give an overview of state-of-the art thin film silicon solar cells and modules using ZnO as transparent front electrode. The chapter ends with a summary, some concluding remarks, and an outlook on future developments.