The recent results of observed mesospheric temperature inversions are presented. Using two Rayleigh lidars (located in the south of France) and ISAMS (Improved Stratospheric And Mesospheric Sounder) and HALOE (Halogen Occultation Experiment) temperature measurements from UARS, a new detailed climatology of the inversions (occurring between 65 and 85 km of altitude) has been developed. A strong annual cycle is observed at midlatitudes, with a maximum during the winter months (monthly mean amplitude of 15 K observed by lidar at 44¿ N, 5¿ E). This annual cycle is also observed by ISAMS and HALOE for all longitudes, so that a midlatitude belt of strong inversions forms during the southern and northern winters. In addition, a strong semiannual cycle is observed by ISAMS and HALOE at lower latitudes, with its maxima 1 month after the equinoxes. Both solstice and equinoctial events are the most significant observed signatures of the inversions. The frequency of occurrence of the inversions follows the same annual and semiannual cycles. The winter inversion events are located about 70 km of altitude, and the equinoctial events are located 5--10 km above, suggesting that different processes could be involved. The winter events observations are consistent with some of the proposed mechanisms, involving the breaking gravity waves, while no evidence of similar mechanisms was found for the equinoctial events.¿ 1997 American Geophysical Union