The goal of this study is to determine if orthotic use impacts the development of walking and upright play skills over and above the impact that treadmill training alone has. Infants with Down syndrome who can pull to stand but not walk will be recruited and assigned to a group that receives treadmill training and orthoses or just treadmill training. The infants are followed monthly until they have one month of walking experience. During the monthly visits, the infants' gross motor development is tested and their upright play behavior is observed. At the end of the study each infants gait is evaluated. We believe that the orthoses will lead to a decreased age at walking onset, an improvement in gait patterns, and an improvement in upright play ability.