Understanding Young Stars: A History

This paper is one of a series of invited reviews celebrating the centenary of the Astronomical Society of the Pacific. The history of pre-main-sequence theory is briefly reviewed. The paper of Henyey et al. (1955), 004.166, is seen as an important transitional work, one which abandoned previous simplifying assumptions yet failed to incorporate newer insights into the surface structure of late-type stars. The subsequent work of Hayashi and his contemporaries is outlined, with an emphasis on the underlying physical principles. Finally, the recent impact of protostar theory is discussed, and speculations are offered on future developments.