All Australian jurisdictions regulate the admission of expert opinion evidence. The rules focus on ‘specialised knowledge’, the existence of a ‘field’, and ‘training, study or experience’. They purport to regulate oral testimony but also the expert reports prepared in advance of potential proceedings, which often shape the way charge decisions are made, pleas are negotiated, and cases are settled, prosecuted, defended and occasionally appealed. In recent decades, in conjunction with attempts to enhance the efficiency of legal proceedings, courts have embarked on efforts to regulate the content and disclosure of expert reports through court rules and practice notes. Most of these emerged from judicial concerns about partisanship and the resources consumed by contested expert opinion evidence in civil proceedings. Only later were they extended to criminal proceedings. This article reviews the rules regulating expert reports, particularly around determining guilt in criminal proceedings. Through a detailed review of responses to an expert certificate in a recent trial in NSW, along with a revised report template developed in its aftermath, this article explains the importance of complying with codes of conduct and practice notes and engaging with scientific research and advice.