The Large Space Systems Technology (LSST) Program is a NASA systems technology program to provide technology which will enable and enhance future space missions requiring large space systems. In the present paper, the rationale for this technology need is discussed. A case is developed by first considering the fundamental opportunities available to earth-orbiting systems to perform needed functions. For these systems, the primary benefits afforded by large dimensions are defined. The impact of the space shuttle on the character of first-generation large space systems is analyzed. Specifically, the effect of shuttle capabilities and limitations on the technical challenges confronting these systems is defined. The paper also outlines current progress, status, and plans for technology development activities of the Large Space Systems Technology Program.