We study Ramsey policies and optimal monetary policy rules in a model with sticky prices and unionized labour markets. Collective wage bargaining and unions monopoly power dampen wage fluctuations and amplify employment fluctuations relatively to a DNK model. The optimal monetary policy must trade-off between stabilizing inflation and reducing inefficient unemployment fluctuations induced by unions' monopoly power. In this context the monetary authority uses inflation as a tax on unions' rents. The optimal monetary policy rule targets unemployment alongside inflation.