This paper reviews the experimental situation in infrared spectroscopy of adsorbed molecules, particularly molecules adsorbed on a single crystalline metal surface. The experimental conditions for this kind of study are defined and some special difficulties with infrared spectroscopy are discussed. This is followed by a review of the different experimental setups that have been used in the field. As an example of an infrared spectrometer dedicated for surface physics work an evacuated wavelength modulation spectrometer is described. Finally, the optimum infrared spectrometer for different kinds of study is discussed