The Bayley Scales of Infant and Toddler Development - Third Edition (Bayley-III) is a widely used psychological test that measures the cognitive, language and motor developmental level of infants and toddlers. The purpose of the present thesis was to investigate cross-cultural differences in Bayley-III test scores, the validity of the language scale, the predictive validity of the test, and gender differences in Bayley-III test scores and test-taking behavior. The results showed that Danish children scored significantly higher on the language scale than American children, and that the three main scales from the Bayley-III differed regarding predictive validity. Furthermore, gender differences in Bayley-III scores were found from 10 months and onwards, with girls in all cases achieving higher scores than boys. The results suggest that when using the Bayley-III in countries other than the US, it is important to be aware that cross-cultural differences can affect test scores. Furthermore, Bayley-III test scores are not necessarily predictive of future developmental levels, and at certain ages, gender may affect test scores.