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Following the initial detection of the Zeeman effect in $\lambda$21 cm HI absorption lines by Verschuur, a program of Zeeman effect observations in HI emission lines was begun with the Hat Creek 85' telescope. This program revealed that magnetic field strengths in the diffuse ISM are usually 10$\mu$G or less. Subsequent studies of the Zeeman effect in $\lambda$18 cm OH lines revealed fields of comparable or slightly greater strength in dark clouds and fields of order 100$\mu$G in the molecular gas in front of several HII regions. Aperture synthesis studies of the Zeeman effect in HI and OH absorption lines have also revealed the presence of 100$\mu$ fields in the vicinities of some HII regions, provided information about the morphology of the field, and yielded indirect evidence for field strengths of order 1 mG. The ensemble of Zeeman effect measurements presently available establishes the importance of magnetic fields in interstellar gas dynamics and suggests that molecular clouds are in approximate virial equilibrium with the magnetic field. Future studies of the Zeeman effect should emphasize higher angular resolution and higher density regions in molecular clouds in order to better delimit the role of magnetic fields in the early stages of star formation.
Tuesday program listing