Sick leave describes an employee’s authorised absence from work due to illness or injury, during which employment continues and specific statutory and contractual rights apply. In the UK, “sickness absence” and “sick leave” are descriptive terms rather than defined concepts, but they underpin entitlement to statutory sick pay (SSP) under the Social Security Contributions and Benefits Acts and related regulations, and any enhanced contractual sick pay. Employers must manage sick leave consistently with employment contracts, absence policies, disability discrimination law, unfair dismissal principles and health and safety duties. In Ireland, “sick leave” is used alongside the statutory concept of “Statutory Sick Leave” under the Sick Leave Act 2022, which sets minimum paid sick leave entitlements, supplemented by contractual schemes and sectoral agreements. Across England & Wales, Scotland, Northern Ireland and Ireland, key issues include: evidence requirements (fit notes or medical certificates), trigger points for managing long-term or repeated absence, reasonable adjustments for disabled employees, protection from dismissal or detriment for genuine sickness absence, and interaction with maternity, occupational injury and insurance benefits.