Business expense describes expenditure incurred wholly and exclusively for the purposes of carrying on a trade, profession or business, and is central to tax deductibility and profit calculation across the UK and Ireland. In England and Wales, Scotland and Northern Ireland, income tax and corporation tax rules (including the Income Tax (Trading and Other Income) Act 2005 and Corporation Tax Act 2009) distinguish allowable business expenses from capital expenditure, private or mixed-use costs, and expenditure with a dual business and non-business purpose. HMRC guidance and case law refine concepts such as “wholly and exclusively”, “revenue” versus “capital”, and “necessarily” for employment expenses. In Ireland, similar principles apply under the Taxes Consolidation Act 1997, which permits deductions for expenses incurred “wholly and exclusively” for the purposes of a trade, with Irish Revenue guidance and case law addressing borderline categories. In legal practice, business expense issues arise in tax planning, corporate transactions, employment and consultancy arrangements, partnership and LLP disputes, and contentious tax enquiries. Key practical questions include timing (accruals vs cash basis), classification (trading vs capital), apportionment where there is mixed use, and evidential requirements (receipts, policies, board minutes) to support deductibility.