Social dialogue constitutes an important part in the process of shaping of social relationships. The importance of social dialogue comes to attention particularly in connection with working conditions for employees. They also have the right to establish different forms of employee representatives so that their economic, social and cultural interests can be promoted and protected with a higher level of dignity and with greater force. A trade union represents one of the fundamental employees’ representatives recognized by law and it also has the right to bargain collectively. Collective bargaining leads to the conclusion of collective agreements that guarantee better working conditions for employees. The prerequisites for operation and action of a trade union within an undertaking differ according to national legislation. The presented article points out the terms and prerequisites of operation and action of a trade union within an undertaking in the scope of the Czech legal order. It brings to attention several application impacts of a trade union’s fulfillment prerequisites of its operation and action, as well as the yet unresolved issue of plurality of trade unions.