Numerous international and national organizations (CIPR 1985, WHO 1988, SFPH-SFRO 1992, AAPM 1994, ESTRO 1994, IAEA 1998, IMRT Collaborative Group 2001…) recommend the practice of in vivo dosimetry to check the global quality of treatments by radiotherapy. This type of dosimetry is much more difficult to carry out for IMRT than for standard treatments because of the numerous and inhomogeneous beams involved (with low dose segments and high dose gradients), and the heterogeneity of dose within the target volume. Atter having recalled the main objectives of in vivo dosimetry (detection of errors and dose verification), a quick review of dosimetric detectors susceptible to answer to problems of IMRT is made. They should be sensitive under a small volume, not connected to high voltage for patient security, with a response accurate, reproducible, independent on dose-rate, beam direction, temperature at least up to 42 °C, and of course, be tissue-equivalent with a low energy dependence. The characteristics, advantages and disadvantages of different detectors such as diodes, MOSFETs, thermoluminescent detectors (TLD), radiochromic films Gafchromic HS and scintillator plastic are reviewed. Considering the particularities of them, it seems that TLD be the best suited for practice of in vivo dosimetry in IMRT. Some. examples of application will be presented to illustrate that. They show that tolerance values different trom those accepted for standard radiotherapy and suited to IMRT is mandatory for a correct exploitation of the obtained The conclusion is that practice of in vivo dosimetry during IMRT is possible but delicate, that TLD dosimetry is the best suited to it but time-consuming and relatively expansive, so that it should be restricted to selected applications in the frame of strict protocols in order to optimize results and cost.