This paper tries firstly to remember the origin of kinetic models in heterogeneous catalysis. The change of catalysis from an art to science induced by the introduction of the concepts of Langmuir is presented in Section 1. In Section 2, deficiencies contained in these concepts as applied, where one site is associated to one chemisorbed radical, are developed. Namely the curious contradictions obtained in the analysis of the hydrogen inhibiting term on the rate of hydrogenolysis reactions are analysed. In Section 3, attention is drawn on information obtained by use of labelled molecules. These studies indicated a similar H-2 inhibiting term on the only adsorption step. The interpretation of that effect results in the development of a multisite adsorption model. Section 4 introduces the interest and advantages of the use of the chemical transient method. Simultaneously, its technical limitations are briefly presented. Sections 5 and 6 are devoted to the application of the transient method to two very different but important catalytic reactions. The first of these examples makes evident the interest in the combination of labelled molecules with chemical transient effects. to determine the rate, direct and inverse, of the successive elementary steps of the model reaction that is ethane hydrogenolysis. The second example, the CO-Hp reaction, is much more complex as the catalytically working surface is built during the transient phase, thus dramatically changing the surface properties. If it does not allow, as in the previous example, to obtain quantitative determinations of the rate of the elementary steps, it anyway provides unique information leading one to propose a new reaction scheme. One of the aims of this paper is to support the still large importance of kinetic studies in heterogeneous catalysis, a topic dear to Michel Boudart with whom one of us (A. Frennet) had the honour and pleasure to work.