Water is cheap, readily available (although only in some parts of the world), non-toxic and non-flammable and is useful for certain types of reaction, in particular bio-transformations and in biphasic processes, in conjunction with other solvents. Phase transfer catalysis, performing reactions at organic-aqueous interface and the use of aqueous surfactant solutions are all examples of successful strategies that can be employed to exploit water as a reaction solvent. However, the use of aqueous reactions can be limited due to issues such as low solubility of organic compounds and incompatibility with a number of reagents. It must not be assumed that aqueous reactions are automatically ‘green’. Treatment and disposal of contaminated aqueous waste can be difficult and can sometimes be very energy intensive.