At mid-twentieth century, the dominant framework for lesson design--the programmed instruction format--was derived from efforts to apply operant conditioning to human learning. Cognitive psychology soon offered an alternative view, represented by Robert Gagne's Events of Instruction, which became the dominant framework through the end of the twentieth century. More recent developments suggest that a new conceptual framework, offered by M. David Merrill, may provide an even more comprehensive synthesis of instructional research and theory than the previous ones. This ERIC Digest discusses the behaviorist model; the influence of cognitive psychology; Gagne's Events of Instruction; constructivist influences; and a new framework by M. David Merrill (2002), "First Principles of Instruction." First Principles proposes four phases to the instructional process--activation of prior experience, demonstration of skills, application of skills, and integration of these skills into real-world activities--with all four phases revolving around a problem. Includes one figure showing the First Principles framework. (Contains 14 references.) (AEF)