Resumen
Outflows are a ubiquitous part of the star formation process and may even be necessary if accretion is to occur onto a protostar or young brown dwarf. While an outflow can take several forms, it is most strikingly seen as a highly collimated supersonic jet that can stretch for several parsecs. Line emission from these jets can be used to derive practically all of their basic physical parameters, such as density, temperature, ionisation fraction, etc. In contrast, their magnetic fields have proven very difficult to measure. Here I review what direct and indirect observational evidence we have that magnetic fields play an important role not only in launching jets but in their subsequent collimation. In the near future, radio observations of any weak non-thermal emission, with instruments such as LOFAR and e-MERLIN, and polarisation studies of the dust component in jets, should help us make estimates of the strength and structure of their B fields.
