This study summarizes the peer-reviewed literature regarding the use of raw pyrolysis liquids (PLs) created from woody biomass as fuels for compression-ignition (CI) engines. First, a brief overview is presented of fast pyrolysis and the potential advantages of PLs as fuels for CI engines. Second, a discussion of the general composition and properties of PLs relative to conventional, petroleum-derived diesel fuels is provided, with emphasis on the differences that are most likely to affect PL performance in CI-engine applications. Next, a synopsis is given of the peer-reviewed literature describing experimental studies of CI engines operated using neat PLs and PLs combined in various ways with other fuels. This literature conclusively indicates that raw PLs and PL blends cannot be used as “drop-in replacements” for diesel fuel in CI engines, which is reflected in part by none of the cited studies reporting successful operation on PL fuels for more than twelve consecutive hours. Based on the reported failure modes, some recommendations are offered for improving performance, reliability, and safety when fueling CI engines with PLs. It appears that PL-derived fuels are most likely to find sustainable CI-engine applications only after a cost-effective pre-use processing strategy is identified to address significant issues regarding fuel instability, materials incompatibilities (e.g., corrosivity), poor ignition quality, high viscosity, and undesirable water/solids/energy contents.