At present, 'gold standard' diagnosis of autism spectrum disorders (ASD) is a lengthy and time consuming process that requires suitably qualified multi-disciplinary team (MDT) personnel to assess behavioural, historical, and parent-report information to determine a diagnosis. A number of different tools have been developed to assist in determination. To optimise the diagnostic procedures, the best diagnostic instruments need to be identified. This study is a systematic review addressing the accuracy, reliability, validity and utility of reported diagnostic tools and assessments. To be included in this review, studies must have (1) identified an ASD diagnostic tool; (2) investigated either diagnostic procedure or the tools or personnel required; (3) be presented in English; (4) be conducted in the Western world; (5) be one of three types of studies [adapted from Samtani et al. in Cochrane Database Syst Rev 3:1-13, 2011], viz. (a) cohort studies or cross-sectional studies, (b) randomised studies of test accuracy, (c) case-control studies. MEDLINE, PsychINFO, Scopus, EMBASE, and Cochrane Library databases were scrutinised for relevant literature published from 2000 inclusive on 20th January 2012. In total, 68 articles were included. 17 tools were assessed. However, many lacked an evidence base of high quality-independent studies. The Autism Diagnostic Interview-Revised (ADI-R) and Autism Diagnostic Observation Schedule (ADOS) stood out with the largest evidence base and highest sensitivity and specificity. When the ADI-R and ADOS were used in combination they revealed levels of accuracy very similar to the correct classification rates for the current 'gold standard' diagnostic procedure viz. 80.8% for ASD. There is scope for future studies on the use of the ADI-R and ADOS in combination.