Although silicon solar cells based on layers less than 50 mu m thick have become very popular, little attention has been paid to the role of the underlying silicon substrate. This treatment uses the device simulation program PC-1D and the ray tracing program SUNRAYS to examine the role of the substrate in contributing to the current and efficiency of textured and non-textured thin layer solar cells. For the case of a heavily doped silicon substrate, substrate contributions can be significant for cells with sufficiently thin base layers. For example, for the case of a silicon thin layer cell with a base layer thickness of 20 mu m and a substrate doping of 6 X 10(18) cm(-3), the substrate contributes no more than 4% of the total short-circuit current. However, decreasing the base width to 5 mu m results in an increase in this substrate contribution to 20%. Light trapping tends to alleviate the substrate contribution by increasing the effective path length in the base. Examination of the current components under forward bias reveals that for a thin layer cell with a high quality base and good front surface passivation, back diffusion of electrons into the substrate limits cell performance.