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 micrometers or less) has been designed, constructed, and tested. The inlet incorporates an inlet configuration allowing for the efficient entry of large particles into the inlet opening, followed by an impactor 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/hr, the maximum wind speed used in the tests. The impactor has also been found to have sharp cut-off characteristics with a sharpness of cut parameter, sigma g, of 1.18. It is believed that this particular inlet will meet the requirements of a high efficiency inlet for sampling inhalable particles from the ambient atmosphere.