The topic of star and protoplanetary disk formation touches almost every area of astrophysics, from galaxy formation to the origin of the solar system. Our understanding of the early evolution of stars has advanced substantially in the last few years as a result of improved observational techniques, particularly in the infrared and radio spectral regions. Although many fundamental problems of star formation remain to be solved, so much has been learned in the last decade about pre-main-sequence accretion processes that an attempt to outline the emerging picture of low-mass star formation seems justified.

In this book I have tried to provide a discussion of accretion in early stellar evolution which can be used at a variety of levels: as an introduction to the subject for advanced graduate students; as a reference for researchers in star formation; and as an overview for scientists in other, related fields. The text assumes a basic familiarity with astronomical concepts and graduate-level physics, though I have made some effort to include some astronomical definitions and references to fundamental physical equations needed for my development. I have adopted a point of view close to that of my own research, which is generally near the interface between theory and observation, and so have tried to discuss basic physical concepts in relation to observational results. Many plausible and even aesthetically pleasing theories have been constructed which have failed to meet observational tests.