The objectives of this study are to 1) demonstrate the effectiveness of the innovative bicycle training program in teaching youth with DS and ASD how to ride a standard two wheel bike, and 2) determine the effects of the treatment on health related performance measures, physical activity level, patterns of sleep behavior, psychosocial outcomes, and community participation. The hypotheses to be tested in this study are: 1) there will be a significant treatment group difference (EXP vs CON) in the number of participants who demonstrate the ability to ride a two wheel bicycle in favor of the EXP group following the first year of training. 2) At the end of the second year of training when the CON group receives the bicycle training program, there will not be a group difference in the number of riders who learned to ride. 3) At the end of the first year, there will be a significant treatment group difference (EXP vs CON) on the health related performance measures, average time spent in moderate and moderate to vigorous physical activity, patterns of sleep behavior (sleep onset, frequency of night waking), self and parent perceptions of bicycle riding ability, and community participation (number of community activities engaged in, frequency of community participation, number of people they participate with outside of the parents, the number of community locations in which they participate), in favor of the EXP group. 4) At the end of the second and third year, the CON group will demonstrate a significant improvement in all of these measures (a significant improvement from the pre-training measures to the 12 and 24 month measures). 5) At the end of the third year, the EXP group will demonstrate a significant group difference on all measures compared to the CON group but the magnitude of the difference will be less than was observed in hypothesis 3 at the 12 month measurement before the CON group received their training. These hypotheses will be tested separately for the DS and ASD groups.