Equivalent fractions are usually introduced in fourth grade and reviewed repeatedly in the subsequent grades as the four arithmetical operations are taught. In spite of this repeated instruction, the results are disappointing. This paper reviews some data from previous research documenting the difficulty of equivalent fractions, explains this difficulty in light of Piaget's theory, and suggests ways in which instruction might be improved. The discussion is limited to fractions of wholes and does not include fractions of sets of objects. Contains 24 references. (Author)