The aim of this thesis was to investigate the relationship between intellectual functioning and sports performance for athletes with intellectual disabilities. A literature review showed that there is a significant difference between physical performance of athletes with and without intellectual disabilities, but so far, no studies have examined the association between the degree of intellectual functioning and sports performance for athletes with intellectual disabilities. Following an analysis of verbal and nonverbal intelligence tests on the basis of their psychometric properties, range of item difficulty, cultural fairness and duration of administration, the nonverbal SON-R 5% - 17 was initially chosen to investigate the relationship between intellectual functioning and physical performance for athletes with intellectual disabilities. The findings revealed that this association depended on sports discipline: for table tennis, scores on a SON-R 5% -17 subtest for inductive reasoning were associated with performance on the ABC physical aptitude test and sports competition performance (ABC physical aptitude test: W adj.= 44%, beta= -.66, p<.01; table tennis competition performance: R2 adj.= 30%, beta= -.66, p<.05), while for swimming and track and field athletics, none of the subtests (nor the overall IQ score) was significantly associated with physical or sports competition performance. However, the results also revealed considerable limitations of this intelligence test for this research: All subtests showed floor-effects (zero scores) and comments from participants indicated that several items contained pictorial representations that were culturally biased. Consequently, it was decided to develop a new computerized cognitive test battery for individuals with intellectual disabilities (CellO), which was focused on the target population (individuals with intellectual disabilities) and based on theories of intelligence, research of cultural fairness, as well as the results from previous testing using the SON-R 5Y2 -17 intelligence test. The test battery included two subtests for inductive reasoning and one subtest for visual processing abilities as these were the subtests shown to be associated with sports performance. Psychometric properties of the CCIID were assessed for individuals with intellectual disabilities using modern and classical test theories. Based on the results of an item analysis using latent trait models and proportion of correctly scored items, several items were revised. A reliability study confirmed internal consistency (r = between .73 and .84, n = 60-66 depending on subtest, p < .05), test-retest reliability (r8 = between .77-.88, n = 24-27 depending on subtest, p < .05) and inter-rater reliability (r8 = between .42-.83, n = 22-25 depending on subtest, p < .05). An exploratory principal component analysis showed one underlying component with an Eigenvalue of 2.04, explaining 67% of the variance. This supported the construct validity of the CCIID. Criterion validity was confirmed based on correlations using Wechsler Adult Intelligence Scale scores (r8= .66, p < .01, n= 18) and scores on the nonverbal SON-R 5Y2 - 17 intelligence test (r8= .82, p < .01, n= 19). Subsequently, the relationship between intellectual functioning and sports performance for athletes with intellectual disabilities was reinvestigated using the CCIID. The results confirmed the findings of the initial studies: for table tennis, scores on the inductive reasoning subtest 'Series' significantly predicted table tennis performance (R2 = .25, beta = .32, p <.05). Again, the studies revealed no significant association between scores on the CCIID and sports performance for swimmers and track athletes. Further research should investigate if sports performance of swimmers and track and field athletes is limited through adaptive behaviour or different cognitive abilities using a wider range of cognitive information processing tests.