Purpose 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. Methods After refining guidelines, 102 video recordings of the SATCo were made on 8 infants with typical development followed longitudinally from 3–9 months of age and 24 children with neuromotor disability mean age 10 yr 4 mo. Eight researchers independently scored recordings. Results ICC values for inter-rater and intra- reliability were > .84 and >.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 .86 to .88. Conclusion The SATCo is a reliable and valid measure allowing clinicians greater specificity in assessing trunk control.