Quality control in a laboratory setting requires the establishment of effective training, standard operating procedures, internal quality control, validation of tests and external quality assessment (EQA). A structured quality management system subject to regular internal and external audits will minimise the error rate. EQA, therefore, gives assurance, both to patients and referring clinicians, that the diagnostic laboratory is competent to produce results that are reliable and accurate. EQA is educational and aims to improve and validate the overall quality of genetic service to the user. Regular EQA assessment compares laboratory performance against set standards and also allows comparison between laboratories. Sometimes EQA can also help to define good standards (best practice), although this does depend on the type of EQA test. EQA interprets best practice standards (=quality) into a numerical score (=quantity). While international bodies or professional organisations set these standards, EQA is able to assess whether these standards are met, with any omissions resulting in a reduction in the total score. Although EQA has an educational role rather than a punitive role, critical errors affecting clinical management will result in a laboratory receiving a poor performance categorisation. Accurate analysis and interpretation are essential quality parameters that require extensive knowledge of the aetiology of genetic abnormalities/disease and risk factors. Training of staff in interpretation of the results together with a comprehensive means of reporting normal and abnormal genetic results underpins the diagnostic service to the patient. Poor-performing laboratories are, therefore, encouraged to review their internal processes. EQA schemes that have been established for many years have seen improvements in the analytical and reporting content over time, thereby improving the quality of diagnostic service available to patients.