The recent sub-millimetre and near-infrared instrumentation allows to spatially resolve the gas and dust emission from circumstellar discs. Especially the dust distribution can be modelled in detail, to reproduce the images and the broad-band
spectral energy distribution.

In this contribution, we investigate how well the spatial distribution and intensity of the molecular gas lines compares to the above models. We present new data on three stars, the isolated Herbig Ae star HD169142, the classical T Tauri star IM Lup, and the weak-line T Tauri star DoAr21. We find that for HD169142 the same model can fit the dust SED and the millimetre CO/13CO/C18O gas emission, with neither freeze out or dissociation of the gas being important.

We deduce a total disk mass of 0.006-0.03 Msun and find limits on the allowed gas to dust mass ratios. By contrast, the CO gas emission around IM Lup is much more extended than the dust (800 versus 400 AU radius). We speculate about possible processes that could have led to divergent evolution for the gas and the dust in the outer regions.

Finally, for DoAr21 we trace the surface of the transitional disk through fluorescent H2 emission at 2.12 μm, resolved with the SINFONI instrument
on VLT. Here we find the gas to reside in a ring extending from as much as 100 to 200 AU from the star, consistent with the lack of dust emission below 20 micron wavelength. The H2 emission is distributed asymmetrically, suggesting that we are either seeing an inclined, warped ring, or are witnessing a global perturbation of the disk. Recent VISIR observations of the warm dust may distinguish between these two scenarios.