Light scattering photometers in combination with monodisperse test aerosols permit continuous recording of the aerosol concentration close to the entrance of the respiratory tract during the whole course of a breathing cycle. Data of this kind together with simultaneous records of the respiratory volumes provide new possibilities in lung function tests. Different inhalation apparatuses based on light scattering have been developed in the last decades. They are classified according to their principle of operation. Requirements with respect to the aerosols are worked out and potential sources of error arising in inhalation photometry are analyzed on the base of experiments. A novel 2-Mode-Photometer is described which can be operated in the photometric or analog mode at aerosol concentrations above 100 particles per cm3 and in the counting mode below 10 particles per cm3. Its applicability for various kinds of aerosol inhalation studies is demonstrated. With this device total deposition on human subjects can be measured at very low number concentrations. Under certain assumptions it can be also used as a sizing instrument for aerosol particles in the respiratory flow immediately at the entrance of the respiratory tract.