Many lithium ion solid electrolytes containing sulfur are not thermodynamically stable against low-voltage anodes like lithium metal. Instead, a Li2S-containing passivation layer forms at the interface, and the transport properties of this layer can limit the overall performance and stability of a solid state battery. Transport in Li2S can also limit the discharge rate of sulfur-based cathodes. Despite this technological relevance, the defect chemistry of lithium sulfide has rarely been explored. Here the ionic and electronic transport properties of Li2S are probed in bulk samples, as well as in thin films prepared by both sputtering and molecular beam epitaxy. One key finding is that the ionic conductivity is higher in thin films than in bulk samples. The origin of this enhanced transport is discussed in terms of doping and interfacial effects. These results help to understand and predict the performance of Li2S-containing passivation layers and sulfur-containing cathodes in solid state batteries.