This article offers an introduction to the central concepts and principles of the Systemic-Structural Theory of Activity (SSTA), an activity-theoretical approach specifically tailored to the analysis and design of human work. In activity theory, cognition is understood both as a process and as a structured system of actions. Building on the general theory of activity, SSTA's use of structurally organized analytical units makes it possible to develop taxonomies and theoretical models of human activity which provide a scientific basis for ergonomic design, education, and industrial-organizational psychology. The primary focus of this article is on design problems in ergonomics. Whereas cognitive psychology has shown a tendency to reduce design problem solving to experimental procedures, systemic-structural activity analyses focus on the interrelation between the structure of work activity and the configuration of the material components of work. SSTA presents methods for the classification and description of human work activity, identifying activity during task performance as the primary object of study, using action as one of the major units of analysis. We outline some applications of SSTA to the study of human work processes, and define and discuss some basic concepts and principles of activity theory. (Contains 2 footnotes, 1 table, and 5 figures.)