Magnetic Fields in Dense Regions

This paper concentrates on reviewing magnetic fields in dense regions; see Heiles (1988, 90) for a review of fields in diffuse regions. The past few years have increased our observational knowledge of magnetic fields in dense regions by an enormous factor—-not because there are many measurements, but because we started from zero. The observable is polarization, which is small and subject to systematic errors. The fact that the advances have occurred only recently is a result of several factors: technological development; the interest and commitment of experimentally-minded astronomers; and the maturing of molecular and infrared astronomy to the point that really new results require either new insights or more difficult techniques