Although constructivism is a concept that has been embraced my many teachers over the past 15 years, the meanings that are attached to this term are varied and often inadequately understood. Teachers need to have a sound understanding of what constructivism means to evaluate its promise and to use it knowledgeably and effectively. This paper explicates some of the theoretical background of constructivism and then presents a detailed example in which a traditional classroom lesson and a constructivist version of the same lesson are described and analyzed. Also discussed are pervasive myths and important instructional issues of this widely advocated and increasingly popular philosophical framework for teaching across the entire K-12 curriculum. Introduction Teachers' personal theories of learning have long been viewed as having considerable influence on virtually all aspects of teachers' decisions about instruction. Not only one's expectations for what learning outcomes are to be valued and sought, but also how one plans (i.e., organizes, structures and sequences) instruction is directly impacted by one's beliefs about learning. In addition, teachers' views of learning guide them as they make decisions about desirable means of implementing and assessing instruction. It is popular today to speak of paradigm shifts, and certainly major conceptual changes do occur in virtually all fields of study. Paradigm shifts bring new perspectives, new conceptualizations and new ways of thinking about a topic, large or small. An important area of study in the philosophy of science is what is referred to as scientific revolutions. Two examples from the natural sciences are the dramatic scientific revolution ushered in by Copernicus' conception of the relationship between the sun and earth, and the revolutionary propositions of Darwin's (though less universally accepted, even today) theory of evolution. When a novel conception is introduced it always elicits great resistance. Even as a transformation in general thinking and attitudes develops more support and adherents, there will continue to be resistance to the challenge to the existing order, the comfortable, existing ways of viewing the world. For example, the ideas of Galileo and Copernicus were met with disdain, anger and rejection. But, of course, with time, the established physical order of the universe did become accepted and the earlier views came to be seen as the quaint notions of an earlier uninformed era. Ultimately most if not all the ideas of the older paradigm will be discarded; and this is as it should be when the scientific evidence unequivocally points to a more adequate explanation of certain phenomena. As a new paradigm gains respect and acceptance, a gradual and sometimes relatively rapid process of intellectual disassociation occurs. People take flight from the earlier, now prosaic and apparently inadequate ways of viewing the world with a lens that is no longer capable of clearly capturing truth. A new, fresh conceptual rendering of a topic, phenomenon or means of investigation is promoted. A new theory is offered to supplant an older theory (Kuhn, 1970). Conceptual change in the social sciences differs somewhat from that in the natural sciences (Thagard, 1992) in large part because the social sciences do not yet have a coherent unifying theory. Thus major conceptual change within a field may better typify significant shifts in the disciplines of the social sciences and education. Nonetheless, the adoption of different theoretical models and application of different assumptions about the nature of human learning has resulted in raging controversies and paradigm shifts within psychology this century (the ascendancy of and subsequent decline of behaviorism; the rise of cognitivism) and in substantial reconceptualizations of philosophy and pedagogy in education. The field of education has undergone a significant shift in thinking about the nature of human learning and the conditions that best promote the varied dimensions of human learning. …