Self gravity, turbulence and magnetic fields (B-fields) certainly all play a role in the star-formation process, which transforms just a small fraction of the mass of molecular clouds into stars. How exactly these forces interact with each other to regulate star formation is, however, still highly controversial. I will report sub-mm/optical observations of B-fields, which can put constraints on the competing models of cloud formation, cloud fragmentation, cloud core formation, and ambipolar diffusion.