This article suggests that it is important to examine how constructivism-in-practice can flourish or flounder according to a variety of influences, noting the importance of outlining the challenges teachers face in understanding and implementing constructivism in classrooms. The article characterizes four major challenges faced by teachers in understanding, developing, and implementing instruction informed by constructivist principles. These challenges are as follows: (1) dealing with the complexity of constructivism as a philosophy, (2) preparing for the subject matter understanding and special pedagogical expertise that constructivist instruction demands, (3) re-envisioning the culture of the classroom, and (4) facing political challenges that arise when implementing constructivist instruction in school settings. The article suggests that articulating the challenges is a significant step in helping educators create and sustain a classroom culture that values diversity in learning and offers a new vision of the roles of teachers and learners. (Contains 37 references.) (SM)