The origin and evolution of large-scale coherent magnetic fields in galaxies remains to be an unsolved fundamental question in astrophysics. Observations of the structure and strength of galactic-scale magnetic fields provide us with clues on the field generation mechanism. In this talk, I will summarize of our current understandings of magnetism in galaxies. I will focus on studies that uses Faraday rotation measures of polarized extragalactic sources to reveal magnetic fields in the Milky Way and the Magellanic Clouds. In addition, I will discuss our on-going effort to measure galactic-scale magnetic fields at cosmological distances using gravitational lensing systems.