The unique properties of magnetic fields in disk-jet systems will be overviewed. Magnetic fields are believed to be amplified within the disk and to emerge to create coronae and/or jets. Recent magneto- hydrodynamic (MHD) simulations have revealed such dynamical behavior of magnetic fields. Their roles are especially significant in radiatively inefficient accretion flow (RIAF), which seem to appear in relatively low-luminosity systems. Finally, we will discuss how MHD flow models fit in the scheme of the alpha-disk models.