The problem of sampling aerosols from the ambient atmosphere has been considered from a theoretical point of view. Following a review of the various samplers and inlets used in ambient sampling, the factors contributing to high sampling efficiency for large particles are discussed. It is pointed out that the major mechanisms for particle loss in sampling inlets are impaction on external surfaces, and impaction, turbulent deposition, and sedimentation on internal surfaces. Therefore, an efficient inlet is one for which these losses are minimized. Based on these theoretical considerations, a new inlet for sampling inhalable particles (particles with aerodynamic diameter of 15 μm or less) has been designed, constructed and tested. The external geometry of the inlet has been selected to allow the efficient entry of large particles into the inlet opening. An impactor is then used to remove the coarse, non-inhalable particles. The inlet has been found to have essentially wind speed independent characteristics for wind speeds of up to 9 km h -1 , the maximum wind speed used in the tests. The impactor used has also been found to have sharp cut-off characteristics with a sharpness of cut parameter, σ g , of 1.18.