Likelihood of confusion describes the risk that the average consumer will mistakenly believe that goods or services come from, or are connected with, another trader. It is a central test in trade mark infringement and opposition proceedings across the UK and Ireland, and also arises in passing off and unfair competition-style claims. In UK law (including England & Wales, Scotland and Northern Ireland), the concept is embedded in the Trade Marks Act 1994 and developed extensively in case law, including decisions of the Court of Justice of the European Union handed down before the end of the Brexit transition period. In Ireland, it is similarly grounded in the Trade Marks Act 1996 and corresponding CJEU jurisprudence. Courts assess likelihood of confusion globally, taking into account visual, aural and conceptual similarity of marks, similarity of goods and services, distinctiveness and reputation of the earlier mark, and the characteristics of the average consumer. Actual confusion is relevant but not required. The concept is broadly consistent across the UK and Ireland, and is crucial in clearance searches, brand selection, trade mark portfolio management, enforcement strategy and litigation risk assessment.