This practice note considers what happens when, after the Central Arbitration Committee (CAC) has issued a declaration recognising a union as entitled to conduct collective bargaining on behalf of a group of workers (the bargaining unit), the bargaining unit changes. If the employer and union are unable to agree a new bargaining unit they may, in certain circumstances, apply for the CAC to consider whether the bargaining unit remains appropriate (and, if not, what it should be) under Part III of Schedule A1 to the Trade Union and Labour Relations (Consolidation) Act 1992 (TULRCA 1992). The employer may also ask the CAC to decide whether the bargaining unit has, in fact, ceased to exist.