This document reports on research in progress on the nature and dimensions of careers. Following a brief introduction, the second section develops and illustrates new concepts and measures for analyzing career lines. The following concepts are defined in terms of the chosen conceptualization: job, career, career lines (regularities in job sequences affecting significant numbers of workers over their work histories), and entry portals (jobs through which a significant portion of the labor force gains access to a career line). Measurement and construction of career lines through data and analysis strategies are discussed in terms of several methodological issues. The illustration of elementary school teachers is used as an empirical example of the career lines that originate with placement as a teacher at age 18-22 in 1965. The third section provides a prospective research agenda (1) for searching for the structure and salient dimensions that characterize the full set of career lines and (2) for establishing the determinants of individual access to careers. Specific topics include dimensions and structure of career lines, determinants of individual access to careers, and design and data for individual-level measures. A sixteen-page listing of references is appended. (YLB)