The Erikson Institute, working with the Ounce of Prevention Fund, the Childcare Network of Evanston and Infant Welfare Society of Evanston, the National Teachers Academy of the Chicago Public Schools, the Carole Robertson Center for Learning, and Erie Neighborhood House investigated the reliability and validity of the Ounce Scale -- a functional assessment of young children's development from birth through 42 months of age. The Ounce Scale is a performance assessment used in Early Head Start programs to monitor infant, toddler, and young preschoolers' development and to guide individualized instruction. It is comprised of three elements: Observational Records, Family Albums, and Developmental Profiles and Standards. The Observational Records provide a structured format for organizing direct observations of children's performance by teachers and caregivers. The Family Albums engage parents in documenting observations of their children's development and provide parents with a range of activities to promote their children's development. The Developmental Profiles and Standards allow staff to evaluate their observations of the children and determine whether the child is "Developing as Expected" or "Needs Development" in six areas of development, based on explicit performance standards. The project utilized extensive multi-method strategies that combined both quantitative and qualitative techniques. The overall design of the project was that of a cross-sectional, concurrent validation. Ratings on the Developmental Profiles were compared to scores on the Bayley Scales of Infant Development-II, the Preschool Language Scale-4, and the Ages and Stages Social-Emotional Questionnaire for 30 children at each of eight age levels (N=240). The sensitivity and specificity of the scale were assessed. Internal reliability and external validity of the scale was determined using Rasch analyses and other Item Response Theory models. The utility of the scale for planning, instruction and facilitating understanding of individual children were evaluated by means of interviews with teachers and parents and through examination of comments in the Family Albums and Observational Records.