Solvent co-intercalation is a process in which ions and solvents jointly intercalate into a layered electrode material during battery charging/discharging. It typically leads to rapid electrode degradation, but new findings show that it can be highly reversible, lasting several thousand cycles. Solvent co-intercalation has two important characteristics: (1) the charge transfer resistance is minimized as stripping of the solvation shell is eliminated and (2) the fact that solvents become part of the electrode reaction provides another means of designing electrode materials. The concept of solvent co-intercalation is chemically very diverse, as a single electrode material can host different types and numbers of solvents and ions. It is likely that many undiscovered combinations of electrode materials, solvents, and ions capable of solvent co-intercalation reactions exist, offering a largely unexplored chemical space for new materials. Co-intercalation can expand the crystal lattice (>1 nm) to the extent that free solvents are present in the structure, forming a layered, “porous” material. This indicates that the concept has a much broader impact and relates to other research fields such as supercapacitors, layered nanostructures, and nanocatalysis. This Review covers the concept and current understanding of solvent co-intercalation reactions, characterization methods, advantages, limitations, and future research directions.