The results of recent researches on twin-fluid atomization are briefly reviewed, with particular emphasis on the effects of air and liquid properties on mean drop size. It is stressed that no single equation for the mean drop sizes produced in twin-fluid atomization can satisfactorily correlate all the experimental data from any given atomizer. This is because for all types of atomizers the mechanism of liquid breakup changes from one basic mode to another with changes in atomizer operating conditions and/or liquid properties. These different modes of atomization exhibit different dependencies on atomizer dimensions and various physical properties such as liquid viscosity and air density. Equations for estimating the mean drop sizes produced in the various modes of twin-fluid atomization are presented and discussed.