Test use is extremely important, not only for clinical practice, but also for scientific research. Nonetheless, some populations have been considered “untestable”. Among the different cognitive abilities assessed using tests, attention is a fundamental one. The present study presents a systematic review of the literature on attention testing in people with Intellectual Disability or Autism Spectrum Disorder, in order to identify: (1) if there are any tests that are fit to assess these populations; (2) which adaptations would be necessary for such tests to become fit; and (3) what limits and needs are involved. Our literature review identified 39 studies (review papers and empirical studies), all of which concern the administration of attention tests for people with Intellectual Disability... or Autism Spectrum Disorder. The selected papers are presented and discussed from two analysis categories: (a) tests, attention abilities, and populations under study; (b) procedures and adaptations made to the testing settings. We identified 72 attention tests, where the majority of the groups of participants in the studies that were analyzed presented mild symptoms. The main adaptations done to the tests refer to strategies used to assist the comprehension of tasks, to communicate instructions, to assure engagement during the procedure, and ways to emit answers. The implications of our results are discussed.