Significant progress has been made over the past two decades in the development of screening and diagnostic instruments for autism spectrum disorders (ASDs). This article reviews this progress, including recent innovations, focussing on those instruments for which the strongest research data on validity exists, and then addresses issues arising from their use in clinical settings. Research studies have evaluated the ability of screens to prospectively identify cases of ASD in population-based and clinically referred samples, as well as the accuracy of diagnostic instruments to map onto 'gold standard' clinical best estimate diagnosis. However, extension of the findings to clinical services must be done with caution, with a full understanding that instrument properties are sample-specific. Furthermore, we are limited by the lack of a true test for ASD which remains a behaviourally defined disorder. In addition, screening and diagnostic instruments help clinicians least in the cases where they are most in want of direction, as their accuracy will always be lower for marginal cases. Instruments help clinicians to collect detailed, structured information and increase accuracy and reliability of referral for in-depth assessment and recommendations for support, but further research is needed to refine their effective use in clinical settings. © 2012 The Authors. Child and Adolescent Mental Health © 2012 Association for Child and Adolescent Mental Health.