Arguments for the use of sequential ordinal scales in the observation of infants and young children are based on the tendency of traditional psychometric assessment to distract investigators from discerning structural and hierarchical aspects of development. Norm-referenced testing focuses on interindividual comparisons rather than developmental patterns. Mental age and IQ scores are often considered to be fixed quantities, and interpreted inappropriately in educational practice. Sequential ordinal scales, as originally suggested by Piaget, invite investigation of structural details. Their advantages are (1) The hypothetical sequential order permits direct testing of hypotheses of the sequential organization of development, (2) They allow for the examination of relationships between developmental patterns and environmental circumstances, (3) They provide means to test Piaget's hypothesis of six sensorimotor stages, (4) They disentangle development from the age variable, (which allows for study of the degree of plasticity in development, and relationships between cognitive competencies and affective conditions). The Uzgiris-Hunt Scales, which contain six series based on behavioral landmarks of development, are described. Research literature in which the Uzgiris-Hunt Scales have been used is reviewed to provide concrete examples of how ordinal scales facilitate investigation of early psychological development. (DP)