The Spanish/English Language Performance Screening (S/ELPS) was designed to assist the preschool and early elementary classroom teacher in objectively determining each child's dominant oral language for initial instruction. This report summarizes the development and testing performed in order to revise and validate the measure. After an intensive design test phase, the instrument was validated by field-testing it with four- and five-year-olds. The first types of external data considered were the responses to questionnaires given to those who administered the S/ELPS. Specific aspects of the measure were tested independently, and the S/ELPS was evaluated by an external consultant. The information collected led to revisions of the entire measure. During field testing, statistical data were collected for validation, and descriptive data on the manual were collected for final modifications. Testing was conducted in schools in San Antonio and Austin, Texas. Formative data were also obtained from Tucson, Arizona and San Diego, Texas. More than 500 children and 25 examiners took part. This report reviews the completed studies and reports the statistical data relevant to the criterion-related validity and reliability. (Author/RM)