ABSTRACTThis article explains the origins of s.4 of the Employment Act 1980 and assesses the place of this provision in the corpus of labour legislation enacted by the Conservative government since 1980. Its principal aim is to concentrate upon the concept of reasonableness which is central to the operation of the provision and to evaluate critically the role of the EAT and the industrial tribunals in interpreting it. Finally, the article seeks to assess the impact of the tribunaľs interpretation of s.4 on union admission and expulsion procedures