The purpose of this study was to compare the usefulness of the Bayley Scales of Infant Development (BS1D)-Mental Scales and the Battelle Developmental Inventory (BDI) in assessing the abilities of infants and toddlers from teen-parent families. The 34 children lived with their mothers, who ranged in age from 15 to 21. The results of the study indicate that these two developmental instruments cannot be used interchangeably and that one needs to look beyond the psychometric data in order to understand the results of infant and toddler assessment. The correlation between the BSID-Mental Development Index (MDI) and BDI Total score was -.03.The United States government has long recognized the need to provide intervention programs and services for preschoolers (Head Start, 1965; Handicapped Children's Early Education Program, 1968). However, lack of trained personnel, inadequate identification procedures and instruments, along with limited funds and facilities have kept services at a minimal level. During the 1980s services to birth to 5-year-olds increased because of additional incentives and mandates to provide services to infants and toddlers, such as PL 99-457, the Education of the Handicapped Amendments of 1986. In addition, various states have decided to serve "at risk" infants and toddlers.Problems persist in the provision of services to this population, especially in the area of assessment. Bagnato (1984) stated that the major dilemma is finding scales that are technically adequate, yet appropriate for the child's functional disabilities, practical for planning intervention, and sensitive for monitoring developmental progress. The psychometric limitations of preschool instruments have been detailed extensively by Bracken (1987). Neisworth and Bagnato (1992) argued that infant and early childhood intelligence tests do not meet social, philosophic, educational, or practical needs, and they advocated the abandonment of such testing. It has never been appropriate to use a measure of intelligence as the sole indicator of overall functioning. Rather, it is extremely important to use the formal and informal techniques that are appropriate for each individual child to gather a large sample of behavior from the child's environment.The professional working with infants and toddlers must attempt to evaluate the assessment instruments and techniques that are available before using them. Some instruments assess one specific area of skill, whereas others cover many. A few instruments have been developed specifically for at-risk children or children with disabilities but the majority have not. The content of the instruments for ages up to 2 years generally consists of simple, readily observable motor actions, whereas the instruments for ages 3 to 6 years usually measure communication, motor, cognitive self-help, and preacademic skills. Excellent and extensive resources are available that list, describe, and evaluate the existing assessment instruments for infants and toddlers, but additional information Requests for reprints ...