Conceptual obstacles which inhibit scientific understanding are examined in this monograph. Perspectives are offered on theoretical aspects, research directions, and educational implications on the problem area known as the critical barriers phenomenon. Papers included are: "Nature of the Problem" (by David Hawkins), which describes the barriers associated with the concepts of size and scale, air and water, heat and elementary mechanics; "The Research Project: Learning about Size and Scale" and "The Research Project: Learning about Light and Color: (both by Maja Apelman), which presents narrative accounts of a program for elementary school teachers on critical barriers; and "Knowing Where You Are: A First Essay Toward Crossing Critical Barriers" (by Philip Morrison), which addresses elements of common sense and its correlations. (ML)