The detailed modelling of thermonuclear runaways in novae is a greatchallenge. This paper focuses on issues where the coupling betweenthe hydrodynamic and the nuclear aspects is critical. The mixing of heavy elements at the bottom of the accreted envelope is found to bethe most significant problem. Other issues related to the modelling of the mixing at the outer parts of the convective zone and to the details of the early light curve are alsodiscussed.