This chapter presents a unified theory of instructional design in the cognitive domain; this includes, of course, online instructional modeling. The theory differs from specific instructional design theories in that it describes how all instructional systems operate (regardless of their goals) in terms of resources and dialogues common to all instructional systems; it predicts certain instructional outcomes (related to groups of learners, not to individual learners) based on given initial conditions. The theory affords practical and theoretical advantages. Practically, it (1) simply and accurately describes the mechanisms at play in instructional systems, (2) presents readily quantifiable operational definitions, (3) suggests hypotheses that may be evaluated empirically, and (4) points the way toward optimizing instructional systems. Theoretically, it (1) subsumes all current theories of instructional design and (2) views campus-based, distance and online instructional systems as a single discipline.