Under the NLRA, the union may demand payment only of those dues for periods when an employee was covered by a collective bargaining agreement that contained a valid union shop agreement. A union shop agreement may not be made retroactive to a period prior to the execution of the agreement. The union may not demand that an employee be fired for failure to pay extraordinary assessments that are not part of regularly, uniformly imposed dues. The LMRDA sets standards for the procedures that the union must follow when asking members to approve an increase in dues.