Abstract: An aerosol generator (10) includes a plate (18) defining an orifice (20) through which a jet of liquid is projected, which breaks up into droplets. The plate (18) is vibrated to control the droplet size, and the size distribution. The droplet size distribution is monitored, and this information is used to control the vibration to ensure the droplets are monodisperse and of the correct size. This can be achieved as follows: the droplets are diverted by a transverse air stream, and an optical image is obtained of the diverted jet (28) onto a linear array of photodetectors (34). The signals from the photodetectors represent the particle size distribution, and are supplied as input to a computer (36) whose output is used to control the vibration of the plate (18).