A deductible business expense is a cost that a business or self‑employed individual may lawfully set against income when calculating taxable profits. In UK tax law, the core test (for corporation tax and income tax) is whether the expense is incurred “wholly and exclusively” for the purposes of the trade or business (see, for example, CTA 2009 and ITTOIA 2005, supported by extensive case law). In Ireland, a similar concept applies under the Taxes Consolidation Act 1997, usually expressed as expenses incurred “wholly and exclusively” for the purposes of the trade or profession.Typical deductible expenses include staff costs, rent, professional fees, and certain finance and marketing costs, but not capital expenditure, most client entertaining or costs with a dual personal and business purpose. The classification of an item as revenue or capital, and the treatment of mixed‑purpose expenditure, are frequent areas of dispute with HMRC and Revenue.Usage and legal effect are broadly consistent across England & Wales, Scotland, Northern Ireland and Ireland, although the relevant statutory provisions differ. Correct identification of deductible business expenses is central to tax compliance, tax planning and advising on corporate transactions, employment structures and self‑assessment.