The injection of water into a spark-ignition piston engine tends to reduce both the power and the tendency to knock. Because of the latter effect the conditions of operation can be made more severe in order to realise overall gains in power and economy. Injection of water at different points in a single-cylinder engine shows the most practicable method to be fine atomization into the inlet manifold Water flow requirements are found to be directly proportional to the manifold absolute pressure, and a commercial type of water injection unit designed on this basis is described and road-tested. An attempt has been made to estimate the distribution of water to individual cylinders of a multi-cylinder engine, and the effect of water up on engine components is examined.