The Segmental Assessment of Trunk Control (SATCo) provides a systematic method of assessing discrete levels of trunk control in children with motor disabilities. This study refined the assessment method and examined reliability and validity of the SATCo. After refining guidelines, 102 video recordings of the SATCo were made of 8 infants with typical development followed longitudinally from 3 to 9 months of age and 24 children with neuromotor disability with a mean age of 10 years 4 months. Eight researchers independently scored recordings. Intraclass correlation coefficient values for interrater reliability were more than 0.84 and 0.98 across all data sets and all aspects of control. Tests of concurrent validity with the Alberta Infant Motor Scales resulted in coefficients ranging from 0.86 to 0.88. The SATCo is a reliable and valid measure allowing clinicians greater specificity in assessing trunk control.