ZnO layers are very frequently used as window layers in photovoltaic solar cell structures. In this application high electrical conductivity σ should be combined with a low value of the optical absorption constant α in the visible range. Thus an appropriate measure of the performance is the ratio r = σ/α and the optimization consists in realizing high by high carrier concentration and mobility with a minimum optical loss due to free carrier absorption at long wavelength. Such layers can be prepared by numerous techniques most frequently by magnetron sputtering either from compound targets or reactively from metallic targets. Optimized doped ZnO-layers have resistivities of 1.5–2×10−4 cm with Hall mobilities of up to 60 cm2/Vs, refractive indices of approximately 2.0 and an average transmittance of 85 % in the visible range. It appears that for the value of μ the physical limit as defined by ionized impurity scattering has been reached for homogeneously doped thick layers. This paper will discuss various aspects of the application of ZnO window layers in photovoltaic solar cells.