Abstract　 　　  

    Star formation is still a mystery. Gravity alone is not able to explain the low star-formation efficiency (~10%) in molecular clouds. While the main-stream models agree on that turbulence can help to support a cloud against self-gravity, the role played by magnetic fields is very controversial. The recent multi-scale observation of cloud magnetic fields in NGC 6334 sheds light on the debate. The field direction does not change significantly from 100 to 0.01 pc, implying that the field is dynamically important compared to gravity and turbulence. In turn, the cloud fragmentation is channeled by the field and self-similar: at multi-scales, elongated cloud structures tend to be perpendicular to the field, which is symmetrically pinched at the ends of cloud elongations. Moreover, the field strength is proportional to the 0.4-power of the density, which is an evidence that the gravitational contraction is channeled by the magnetic field.