Supramolecular reactions between crystalline materials or reactions between a crystalline material and a vapour can be used to generate new crystalline substances as well as crystal polymorphs, solvates, salts and co-crystals. These solvent-free processes can be exploited to prepare both hydrogen bonded co-crystals and coordination networks by non-solution solvent-free methods. Solid-vapour reactions do not differ from solid-gas uptake/release processes, and can also be used to prepare polymorphs and solvates. It is argued that solvent-less reactions involving molecular crystals represent a green route to supramolecular solid state chemistry and a viable alternative to solution methods for a thorough mapping of the "crystal space" (crystal reactivity, stability, vapour uptake/release, behaviour under mechanical stress, etc.). Examples will be provided in a broad range from organic molecular crystals, to coordination networks and salts.