Blood-alcohol content (BAC) describes the concentration of alcohol in a person’s bloodstream, usually expressed as milligrammes of alcohol per 100 millilitres of blood, or as a percentage. It is a central evidential concept in drink-driving offences and related road traffic prosecutions across England and Wales, Scotland, Northern Ireland and Ireland. BAC is not usually a defined term in its own right, but statutory limits and evidential rules are set out in legislation, including the Road Traffic Act 1988 (England and Wales, Scotland), the Road Traffic (Northern Ireland) Order 1995, and the Road Traffic Act 2010 (Ireland). The legally prescribed limit for drivers differs by jurisdiction (and, in some cases, by driver category), so practitioners must check the specific statutory provisions. BAC is typically established through evidential breath, blood or urine tests, subject to procedural safeguards and potential challenges (for example, reliability of equipment, chain of custody, medical issues). BAC levels are also relevant in criminal and civil litigation when assessing impairment, contributory negligence, or capacity, and may inform sentencing, insurance coverage disputes and employer disciplinary processes. Usage of the expression “BAC” is broadly consistent across the UK and Ireland.