It is widely believed that magnetic fields play a crucial role for the dynamics of molecular clouds and for the star formation processes. One of the most informative techniques of studying magnetic fields in molecular clouds and protostellar disks is based  on the use of polarized emission arising from magnetically aligned dust. In this talk, I will talk about the Chandrasekhar-Fermi (CF) method, which is a simple and powerful technique to measure magnetic field strength from FIR/sub-mm polarization observations. I will demonstrate that the conventional CF method tends to overestimate the strength of the magnetic field and describe how to correct the tendency. When time permits, I’ll briefly talk about polarized emission from T Tauri disks.