Parent- and lab-based observer ratings were employed to examine genetic and environmental influences on continuity and change in inhibitory control (IC) in over 300 twin-pairs assessed longitudinally at 2 and 3 years of age. Genetic influences accounted for approximately 60% of the variance in parent-rated IC at both ages. Although many of the same genetic effects on parent-rated IC were stable across age, there were also novel genetic effects that emerged at age 3 (i.e., genetic factors contributed to both continuity and change in parent ratings of IC). Observed IC displayed a different developmental pattern. Genetic influences were moderate at age 2 (38%) and nonsignificant at age 3 (6%). Change in observed IC across early childhood was due to shared and nonshared environmental factors. Findings indicate that it is important to consider the measurement of IC when interpreting developmental and etiological findings.