Critical use of investigations is one of the core skills of Australian general practice training and previous research has demonstrated that this is a challenging area for GP registrars. Inexperience and intolerance of uncertainty can lead to overtesting, with consequent risks of ‘investigation momentum’, false positives, ‘incidentalomas’ and patient harm. GP supervisors play a key role in influencing GP test ordering behaviour. This tutorial is linked to the supervision activity of TRAFk (test result audit and feedback – see GPSA Guide).