The purpose of this study was to develop a valid and reliable tool for use in assessing motor skills in preschool children in field-based settings. The development of the Children's Activity and Movement in Preschool Study Motor Skills Protocol included evidence of its reliability and validity for use in field-based environments as part of large epidemiological studies. Following pilot work, 297 children (3-5 years old) from 22 preschools were tested using the final version of the Children's Activity and Movement in Preschool Study Motor Skills Protocol and the Test of Gross Motor Development (2nd Edition). Reliability of the Children's Activity and Movement in Preschool Study Motor Skills Protocol and interobserver reliability were determined using intraclass correlation procedures (intraclass correlation coefficients; ANOVA). Concurrent validity was assessed using Pearson correlation coefficients to compare the Children's Activity and Movement in Preschool Study Motor Skills Protocol to the original Test of Gross Motor Development (2nd Edition). Results indicated that test reliability, interobserver reliability, and validity coefficients were all high, generally above R/r = 0.90. Significant age differences were found. Outcomes indicate that the Children's Activity and Movement in Preschool Study Motor Skills Protocol is an appropriate tool for assessing motor development of 3-, 4-, and 5-year-old children in field-based settings that are consistent with large-scale trials. (Contains 6 tables.)