In this paper, the author reviews the existing literature on aptitude testing with a view to highlighting the main emerging themes: which qualities indicate an aptitude in a prospective interpreter, how these qualities may be measured and which types of test should be administered, the issue of valid and reliable testing, proposals for test designs, and, finally, description of aptitude tests which have identified statistically significant predictors. The focus is on spoken language, but signed-language aptitude testing is also partially covered. Available results so far appear to show that interpreting-related cognitive skills and verbal fluency may be measured and may be predictive both for spoken-language and for signed-language interpreting candidates. In particular, the production of synonyms appears to be a strong aptitude predictor from several independent research projects.