Composites have historically been of intense interest due to their potential to replace glass, wood and metal at a fraction of the weight. Of the many choices of fillers, graphene oxide (GO) is generally attractive for its versatility and tunability in terms of surface functionality, size and shape. While the majority of GO has been incorporated into polymers and polymer precursors by physical mixing, this review focuses on research where GO has served as both a property enhancer and multifunctional crosslinker in thermosets. Methods for preparing GO (with and without additional functionalization) and incorporating it appropriately into thermosets are described. A review of characterization techniques, typically applied before and after GO is incorporated into thermosets and analytical methods for confirming important chemical reactions during crosslinking, is also provided. Finally, the resulting composite thermoset properties are surveyed throughout to connect preparation and characterization methods to their potential practical importance.