rare objects are in many ways the intermediate “missing link" between the cold atmospheres of the Solar System’s giant planets and cool stellar atmospheres. The physics and chemistry of their atmospheres, dominated by broad molecular absorption bands, are very planetary-like [see @Kulkarni.1997 for instance] and the cool brown dwarfs spectra are the key to constraining planetary and stellar atmosphere models. Nowadays, the Teff$<$700K atmosphere temperature regime is troublesome for modellers. A few ultracool late T brown dwarfs have now been discovered with effective temperatures below 650K. These discoveries step into unexplored territory and a new generation of models is emerging. It is facing several difficulties. 1) Out-of-equilibrium chemistry plays an important role, resulting for instance in NH$_3$ being less prevalent than expected [@Cushing.2006]. 2) Fine details of convection control both the L/T transition and the dredge up of hot chemical species in late T atmospheres. 3) Water cloud formation and dust nucleation play important roles. 4) Line opacities of several molecules, in particular NH$_3$ and, to a lesser extent, CH$_4$ are unknown and cause important spectroscopic feature mismatches. As a good example of the need for refined models, @Burningham.2009 have recently determined that a T8.5 dwarf companion to an M 