Competence to stand trial refers to a criminal defendant’s capacity to meaningfully understand proceedings, assist counsel, and make decisions during adjudication. Competence is crucial to fair criminal proceedings, preserving both the dignity and reliability of the court, but also the dignity and autonomy of defendants. The principles underlying a competency requirement have been long established, and formally recognised in the United States under the Dusky (1960) standard, which remains the basis of all state standards. Incompetency must be based on a mental illness or disability, usually symptoms of psychosis or cognitive intellectual disability. Yet no particular diagnosis equates with incompetence. Rather, competence is functional and contextual and depends on whether the defendant manifests certain basic abilities (i.e. understanding, reasoning, assisting counsel, and making decisions) sufficient to the demands of the defendant’s particular legal case. Evaluations of trial competence are the most common type of forensic mental health evaluation conducted, and data suggests that referrals for competency to stand trial evaluations are increasing. Historically, research suggested that around 20 per cent of defendants referred for competence evaluations were opined to be incompetent, but this rate too may be rising, at least in many United States jurisdictions. Research has commonly found that incompetency is strongly associated with severe psychiatric conditions (i.e. psychotic and cognitive disorders), but less research has addressed the more precise symptoms causing incompetence. Finally, field studies have revealed moderate to good reliability in ‘real-world’ evaluations but reveal some room for improvement. Although the procedures surrounding competency to stand trial evaluations are well-established, the field has room to further improve best practices in reliable assessment, competency restoration, and the public mental health policies addressing competence evaluation and restoration.