Recent CO surveys and measurements of the magnetic field intensity and direction have renewed the description of molecular clouds. They appear as a large self-similar hierarchy of structures with a unique power law mass spectrum over 9 orders of magnitude in masses. No lower threshold has yet been observed for this hierarchy in non star forming clouds. At the opposite, in clouds which have started to form stars, the self-similarity breaks down at the scale of the dense cores. The whole hierarchy is fractal. Molecular clouds appear to be in virial balance. The magnitude of the external pressure terms is large in non star forming complexes and very likely due to magnetic stresses. The need for an external pressure decreases as the star formation activity increases and the equality 2T+Ω=0 is reached only for the most massive substructures of GMCs, on the verge of (or already) forming stars. The gas motions are turbulent and likely trans-Alfvénic, given the observed magnetic field intensities. The whole hierarchy of scales is characterized by an invariant, ρvl3/l, the transfer rate of kinetic energy. Molecular cloud cores appear to be supercritical and a static magnetic field cannot stabilize them against gravitational instability. MHD turbulence captures several of the characteristics of molecular clouds and is able, as long as it is fed, to prevent clouds from collapsing. Its dissipation, alike in hydrodynamical turbulence, is concentrated in bursts, which are sheetlike structures.