A systematic review of literature in the area of counting in Down syndrome was conducted to identify and analyze ability to count of children with Down syndrome. We firstly reviewed the most famous theories which have explained how typically developing children acquire counting, and then we discussed how children with Down syndrome acquire counting according to these theories. We showed how children with Down syndrome have a deficit in counting and demonstrated the main reasons which may lie beneath this difficulty. Inspite of this difficulty in counting, we found that children with Down syndrome benefited from intervention. We ended the review by briefly summarizing the characteristics of good interventions to demonstrate how we can improve children with Down syndrome's ability to count.