Abstract : The Mars Express payload includes a suite of instrument which renew our view of the Mars Climate System. To make themost of these combined observations and help to interpret them in the general context of Mars Climate Science, we will compare these observations with simulations from our Global Climate Model (GCM) developped at LMD in collaboration with the University of Oxford and IAA in Granada. This model includes a state of the art parameterization of the water, CO2 and chemical cycle, and has recently been extended into the thermosphere. This talk will include the following aspects: 1) An analysis of the thermal structure as observed with the various instruments, in particular with Spicam stellar occultations, PFS inversion of the 15 microns band and MaRS radio-occultation temperature profiles. 2) An analysis of the composition of the polar caps (water, CO2, dust components, grain size) as observed by Omega in particular. The model can simulate the evolution and the stratification of these various components, and it helps to shed light on the physical process at work. 3) A comparison with the observed measurements of the water vapor. The different wavelengths observed by the Mars Express instruments can provide different values, sometime contradictory. With the help of the model we can try to reconcile these observations and possibly help to identify some new sources. 4) Tranport of tracers by the martian atmosphere. Among the surprising results from Mars Express are the observations of methane and other trace gases exhibiting spatial variations. We have performed theoretical observations with various lifetimes, sources and sinks to better constrain the nature of the processes affecting these gases.