Research has suggested that in typical developing children a positive relationship exists between physical activity level and cognitive functioning. For some children, academic performance may increase when levels of physical activity are increased. Moreover, some studies have supported the idea that physical activity seems to improve attention. Working memory, which is involved in the processing, storing, and manipulation of information, is linked to both IQ and achievement. Studies have shown that children with poor working memory do not perform well academically. Decreased levels of physical activity levels and sedentary behaviors are linked to the increasing obesity rate in the United States. Research on children with intellectual disabilities has found lower rates of physical activity and higher rates of obesity. This study investigated the relationship between physical activity, Body Mass Index (BMI), and working memory components in a sample of 25 children and youth with Down syndrome to evaluate whether physical activity and BMI can predict working memory performance. Furthermore, this study assessed whether higher reported physical activity and lower BMI would translate to better working memory components scores. Findings from a multiple regression were not statistically significant but point to the fact that lower BMI and higher reported physical activity can explain variances in working memory in this sample. Effect sizes indicate that for the intellectually disabled (ID) Moderate/Severe group in the sample both reported physical activity and BMI account for moderate effect sizes in the four components of working memory. The largest effect size is found for visuo-spatial short term memory accounting for 39% of variance. For the children in the ID Mild category, a large effect size is found for age: working memory continues to grow from the age of 10-18. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]