Ball mill grinding reactions can be performed under neat conditions (NG) and under liquid assisted grinding (LAG) conditions, wherein a sub-stoichiometric amount of liquid is added to the powder. This addition of liquid often leads to faster reactions and to a different outcome as in the formation of a different polymorph of the product. Polymorphism is a property inherent to the solid state. It is the ability of a compound to exist in multiple solid forms differing in relative intermolecular and/or interatomic distances. Such crystal properties determine physicochemical properties such as melting point, dissolution behaviour, solubility, reactivity, bioavailability and stability against physical and chemical stress, making the topic of understanding how different polymorphs can be reliably prepared of major interest especially to the pharmaceutical industry. Many cocrystals and salts that are inaccessible by solution methods have been prepared by ball mill grinding; in some cases various polymorphs have been reported. In most cases only two polymorphs have been prepared by ball mill grinding. Only a handful of examples of the full or partial preparation of three polymorphic forms of the same substance (crystalline or amorphous) by ball mill grinding have been published to date: anthranilic acid, γ-aminobutyric acid (GABA), N-acetyl-l-phenylalanyl-amino, Praziquantel, 1:1 caffeine-anthranilic acid cocrystal, and 1:1 salt of vinpocetine oxalate. Polymorph conversion by ball mill grinding has been an area of interest, as it enables the formation of bulk quantities of metastable polymorphs by alternative techniques to traditional ones, though often not accessible by classical solution methods. Recently, Arhankelskis et al. demonstrated reversibility between two crystalline and one amorphous form of the 1:1 salt of vinpocetine oxalate. Only partial reversibility of polymorph transformation between pairs of polymorphs has been shown in those cases where three crystalline polymorphs were obtained by ball mill grinding. Full reversibility between two polymorphs by ball mill grinding has been shown for a number of molecular compounds, via "one pot polymorph turnover experiments". In these experiments, the same sample powder is converted from one polymorph to the other and back by changing the milling conditions. This means that the milling jar is opened to allow for the LAG solvent to be removed by evaporation and for analyses to characterize the milled powder, but the powder itself is not replaced. We believe that full reversibility of polymorph conversions by mechanochemical methods will prove to be a common and general phenomenon. For this reason, there should be no limit in principle to the number of polymorphs that can be reversibly transformed by such turnover experiments.