The use of IR spectroscopy for the evaluation of the Lewis and Bronsted acidity of microporous systems is illustrated having recourse to examples concerning zeolites, heteropolyacids and sulfonated membranes (NAFION). Methods based on the perturbation of the OH modes by interaction with basic probe molecules are illustrated as well as on the perturbation of the internal modes of the probe itself. The use of H2 as probe is also debated by discussing new data specifically obtained for this review. The illustrated case examples are mainly obtained from the experience gained by the Turin group in 20 years.