A new aerosol particle classiﬁer, the Aerodynamic Aerosol Classifier (AAC), is presented, with some of its applications. The instrument uses a centrifugal force and sheath flow between two concentric rotating cylinders to produce a truly monodisperse aerosol classified by aerodynamic diameter. The instrument is unique in being able to produce a stream of particles of known finite aerodynamic diameter; other technologies such as impactors and virtual impactors provide either an upper or lower cut. Since the classification technique is independent of particle charge state, a true monodisperse aerosol is produced without any multiple-charging artefacts. This work reports theoretical and experimental results of this new instrument, including the accurate classification of particles down to ~20 nm in aerodynamic diameter.