This study compared the functional efficacy of three commonly prescribed ankle–foot orthosis (AFO) configurations (solid [SAFO], hinged [HAFO], and posterior leaf spring [PLS]). Sixteen independently ambulatory children (10 males, six females; mean age 8 years 4 months, SD 2 years 4 months; range 4 years 4 months to 11 years 6 months) with spastic diplegia participated in this study. Four children were classified at level I of the Gross Motor Function Classification System (GMFCS; Palisano et al. 1997); the remaining 12 were at level II. Children were assessed barefoot (BF) at baseline (baseline assessment of energy consumption was performed with shoes on, no AFO) and in each orthotic configuration after three months of use, using gait analysis, oxygen consumption, and functional outcome measures. AFO use did not markedly alter joint kinematics or kinetics at the pelvis, hip, or knee. All AFO configurations normalized ankle kinematics in stance, increased step/stride length, decreased cadence, and decreased energy cost of walking. Functionally, all AFO configurations improved the execution of walking/running/jumping skills, upper extremity coordination, and fine motor speed/dexterity. However, the quality of gross motor skill performance and independence in mobility were unchanged. These results suggest that most children with spastic diplegia benefit functionally from AFO use. However, some children at GMFCS level II demonstrated a subtle but detrimental effect on function with HAFO use, shown by an increase in peak knee extensor moment in early stance, excessive ankle dorsiflexion, decreased walking velocity, and greater energy cost. Therefore, constraining ankle motion by using a PLS or SAFO should be considered for most, but not all, children with spastic diplegia.