This book contains the contributions to a workshop on interstellar magnetic fields held at Ringberg Castle in September 1986. It covers results from a very active field of astronomical research which owes its existence to the significantly improved detection of the polarisation of light and radio waves from within our Galaxy or its neighbors. Theories of the origin of interstellar magnetic fields are presented together with new observational results on the influence of these fields on the interstellar medium, star-forming regions and supernova remnants. First results of magnetohydrodynamics calculations show the relevance of interstellar magnetic fields on jets and accretion disks.