[en] The measurement of radon in underground cavities is not codified by a normative text. However, a specific methodology has been developed by I.R.S.N. in the area of studies devoted to the characterization of radon content in different underground cavities. These underground cavities can be classified in three categories: firstly, the natural cavities descended from the dissolution of carbonates or sulfates by water circulation. secondly, anthropogenic spaces, out of mines (underground quarries relative to the materials extraction for construction, marl pits, basements, civil construction works (tunnels, aqueduct) and military ones (war sapping), thirdly, the mines (coal, metals, salts). The present document presents the methodology applicable to the radon detection in a natural underground cavity. It is founded on the approach initiated by the AFNOR NF M60-771 norm for the realisation of detection and on the results of studies realised by I.R.S.N. in the karstic caves and the french agricultural underground cavities. The implementation of this methodology requires knowledge relative to radon and cavities. It is thus the responsibility of relevant agencies. (N.C.)