Recent CO surveys and measurements of the magnetic field intensity anddirection have renewed the description of molecular clouds. They appear as alarge self-similar hierarchy of structures with a unique power law mass spectrum over 9 orders ofmagnitude in masses. No lower threshold has yet been observed for thishierarchy 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 thedense cores. The wholehierarchy is fractal. Molecular clouds appear to be in virialbalance. The magnitude of the external pressure terms is largein non star forming complexes and very likely due to magneticstresses. The need for an external pressure decreases asthe star formation activity increases and the equality 2T+Ω=0 is reached only for the most massive substructures ofGMCs, on the verge of (or already) forming stars. The gas motions are turbulent and likely trans-Alfvénic, given the observed magneticfield intensities. The whole hierarchy of scales is characterized byan invariant, ρvl3/l, the transfer rate of kinetic energy. Molecular cloud cores appear to be supercritical and a static magneticfield cannot stabilize them against gravitationalinstability. MHD turbulence captures several of the characteristics ofmolecular clouds and is able, as long as it is fed, to prevent cloudsfrom collapsing. Its dissipation, alike in hydrodynamical turbulence,is concentrated in bursts, which are sheetlike structures.