The major purpose of this study was 1) to investigate the development of the concept of a unit of measure and the coordination of unit size and the number of units 2) to relate this development to the development of conservation and 3) to determine the role of equivalence and nonequivalence relations in certain conservation and measurement problems. 218 subjects (grades K-2) were individually tested on items selected from a set of conservation and measurement problems. In one set of measurement problems, quantities were measured with different size units of measure. Centering on either unit size or the number of units alone lead to errors similar to those found in conservation problems. In a second set of measurement problems, quantities in different shaped containers were measured with the same unit. Each type of problem was administered in three situations employing different combinations of equivalence and nonequivalence relations. Results are discussed in detail. (Author/MS)