We present a statistical study on tropospheric layers as allowed by the most extensive ozone and water vapor database currently available. Considering O3 and H2O deviations from an automatically calculated background, we define four types of layers. These tropospheric layers are a common feature, with the percentage of the troposphere occupied by such layers varying from 7% to 33% depending on the region and the season. Most of the layers are found between 4 and 8 km altitude, and the median thickness is about 500 m. At northern midlatitudes we find 4 times more layers in summer than in winter, while in tropical Asia we observe a spring maximum in the occurrence of the layers. The most abundant layer type everywhere is O3+H2O- and corresponds to the signature of stratospheric intrusions or continental pollution. This suggests that stratosphere-troposphere exchanges or at least their influence are not negligible in summer at midlatitudes or in the tropics. A complete understanding of the layers could lead to a better empirical assessment of the different tropospheric ozone sources and to an assessment of the potential vorticity fluxes in the troposphere. ¿ 2000 American Geophysical Union