Abstract: A method and an apparatus for measuring the wall thickness of a plastic article, particularly of a hollow rotation-symmetrical plastic article. According to the method, the wall of the plastic article is subjected to infra-red radiation and the intensity of, e.g., the continuously transmitted radiation is measured at preselected points, said intensity being a function of the wall thickness to be measured. At the same time, on one hand, the plastic article and, on the other hand, a radiation source and/or a radiation detector are set in an axial and rotational movement in relation to each other.