It is possible to improve the performances of gas sensors using filtering films déposited above the sensing marerial. This paper is a review of several rencent studies performed by the authors in this sense in order to point out the major dificulties on this subject. The difference between thin and thick film technologies is well underlined. Firstly, an example of thick porous layer is presented with the objective to develop a parous protective layer for soot protection in car exhausts. By another way, the use of metallic films has been successfully used to improve the selectivity of SnO2 sensors. Even if thin films of rhodium allow to solve the problem of the dual response CO/NO2, the best results are obtained generally with thick films. This is partly due to several problems resulting of the direct contact of the metallic films with the sensing material. The use of SiO2 thick insulating layer allows to improve the selectivity to CH4 with a fully agreement wirh catalytic activities of a platinium filter in regards the CO interference. Alast example demonstrates also the interest of thin SiO2 films for the selective detection of H2.