This session will explore the magnetic fields of planets, extra-solar planets and cool stars, at a time of rapid advancements in this area. The magnetospheres of planets such as Jupiter and Saturn have been studied both by in-situ observations of their magnetospheres and through auroral emissions. Although much has been learnt about planetary magnetospheres, many questions remain unanswered, some of which will be addressed by the upcoming Cassini Grand-Finale mission at Saturn and the Juno mission to Jupiter. With the improvements in instrumentation and data analysis techniques, magnetic fields can now be detected and studied at ultra-cool dwarfs, which have surprisingly been revealed as potential analogs of planets in their manifestation of magnetic activity by the emission of bright radio bursts of a similar nature to auroral planetary radio bursts. This session will focus on the observations of magnetic fields, using in-situ and remote sensing within our solar system, to techniques available to detect exoplanetary and ultra-cool dwarf magnetic fields, as well as associated theoretical studies. Discussion will focus on how best to bridge our understanding of activity across the mass gap from planets to cool stars. To facilitate this, we plan to hold a half-hour panel discussion as part of the session.