Solid-state lithium-ion batteries promise high energy density and excellent safety but there are many challenges to overcome before these goals can be realised. Crucial to the operation of solid-state batteries is the solid electrolyte, which must be able to compete with more traditional liquid electrolytes with regards to ionic conductivity. Solid electrolytes are often produced through the sintering of powders, yielding structures in which grain boundaries (GBs) are highly prevalent. GBs should be expected to have profound impacts on the performance of a device throughout its lifetime, some of which can be positive, such as the observed increase in ionic conductivity of Li 3 PS 4 containing nanosized grains. However, GBs are generally considered to be performance bottlenecks in solid electrolytes and can cause decreased ionic conductivity and even device failure through the propagation of lithium dendrites, leading to short circuits. Clearly, an in-depth understanding of the impact of GBs on solid electrolytes is essential, but given that there is a huge variety of candidate electrolyte materials, this is a not a trivial feat. The problem is further complicated by the fact that the interpretation of experimental results for polycrystalline materials can be difficult, which makes computational techniques invaluable for providing insight at the atomic scale. Despite this fact, there is currently an alarming lack of atomistic computational studies of GBs and their effects in solid electrolytes, particularly regarding their impact on electronic structure. Some progress has been made regarding ion transport at solid electrolytes GBs using molecular dynamics studies with classical potentials. For example, it has been established why oxidebased solid electrolytes generally exhibit higher GB resistance than sulfides and how GBs provide an explanation for the often observed differences in calculated and experimental Li-ion conductivities in some solid electrolyte families. Although classical models are valuable as their low computational expense enables the investigation of large grain boundaries and nanostructures containing many tens or even hundreds of thousands of atoms over long timescales, their utility is limited by the fact that they lack access to electronic properties. Electronic properties are of increasing interest in the fields of solid electrolytes and solidstate batteries as it is these properties that underpin undesirable electrical conductivity and, especially at GBs, dendrite formation. In this study, we utilize first-principles calculations to elucidate how GBs impact the performance and properties of four archetypal solid electrolyte materials, namely, an oxidebased anti-perovskite (Li 3 OCl) and its hydrated counterpart (Li 2 OHCl), a thiophosphate (β-Li 3 PS 4 ) 8 and a halide (Li 3 InCl 6 ). The GBs in all four materials are found to exhibit narrowed band gaps and charge-trapping states that would be conducive to unwanted electronic conductivity via polaron hopping and potential dendrite formation. By comparing the results of ab-initio molecular dynamics (AIMD) simulations on the bulk and GBs of each material, it is found that the Li 3 OCl GBs pose large barriers to ionic conductivity whereas the GBs in Li 2 OHCl, Li 3 PS 4 and Li 3 InCl 6 are comparatively benign. While the contrast between GB resistance in oxides and sulfides is generally accepted, this study is the first to demonstrate the nature and origin of GB resistance in a halide-based solid electrolyte. We also show the significant influence that GBs have on correlated Li-ion transport for the first time. Our results provide design principles and important considerations for the synthesis and doping of solid electrolyte materials with high ionic and low electronic conductivity.