Research on the concept of teacher efficacy spans over 20 years, but much remains to be learned. Although precise definitions of the concept have always been problematic, in general, teacher efficacy is defined as teacher's belief or conviction that they can influence how well students learn (T. Guskey and P. Passaro, 1994). Efforts to clarify the definition of teacher efficacy are sometimes clouded by similar or related constructs. It is suggested that the only major difference between perceptions of efficacy and responsibility is in the tense of the items used in the measure, with efficacy representing projected potency and responsibility being an attribute directed toward the past. From the earliest research, teacher efficacy has been considered to have two dimensions, sometimes suggested to be outcome expectations and efficacy expectations. Others have interpreted the dimensions as personal efficacy and teaching efficacy. Guskey and Passaro (1994) have found the two dimensions to be: internal, the extent that teachers believe that they, and other teachers, have the influence and impact on student learning; and external, a dimension that measures teachers' perceptions of the influence and control of factors outside the classroom. This distinction is not the same as locus of control, because these two factors operate fairly independently. Researchers have identified other factors that may be equally powerful and important, and these remain to be studied in detail. (Contains 16 references.) (SLD)