This 4-year longitudinal study of logical reasoning found complex interrelationships among different cognitive processes of children ages 6 to 15. Piaget's stage theory is discussed in the introduction, with a focus on the concrete operational stage in middle childhood. In the study, a representative array of logical concept tasks and short-term memory measures was administered to children at the kindergarten, third, and sixth grade levels. Of the initial sample of 134 subjects, 88 participated in all four years. The target domains included relations (e.g., seriation), classification (e.g., class inclusion), natural number (e.g., number conservation) and conservation (e.g., equivalence of weight), and the groupement structures into which they enter. The relative difficulty of the tasks, age-task and other interactions, and the connections among the abilities involved are described statistically. It is suggested that the data fit the sigmoid sequence (a series of ascending curves) of stage transitions proposed by Feldman and Toulmin, in which some skills develop in a lock-step fashion and others relatively independently of one another, better than the classical model of stepwise transitions. It is concluded that the primary research question concerns the modifications necessary in the Piagetian concrete operations stage designation to permit it to play a continued role as a potential explanatory construct in more complete accord with developmental reality. Data are presented in 39 tables (comprising nearly half of the report), and there are 14 figures representing grade or age by performance means for the different tasks. (Author/BH)