The present paper has described the considerations that led to the formulation of a longitudinal study of child development. It has indicated that such study is necessary for: 1. The determination of the variables which contribute to personality formation, 2. The understanding of individuality in psychological functioning, 3. The relationship of behavioral disturbance to psychodynamic and non-psychodynamic etiologies, and 4. The importance of longitudinal study of a child for parental guidance.The data presented have demonstrated the existence of stable, primary patterns of reactivity in children. These data were derived from parental interview and a variety of independent direct observational and interview sources. The primary patterns defined have been considered with respect to their early appearance in infancy, their perisistence in childhood, and their pertinence for the emergence of adaptive patterns in children. It has been concluded that initial primary reactivity is a crucial variable together with environmental influences in shaping both personality structure and temperament.