The study aims to empirically study the Early warning system and how national parliaments in the EU vary regarding the issuing of reasoned opinions. In recent years the subject of subsidiarity within the EU has become an increasingly debated and controversial topic. The Early warning system aims to alleviate the democratic deficiency of the EU, but the scientific community is divided on how efficient it is. Using data from the European parliament this study examines reasoned opinions and factors that influences why national parliaments issue them and why there is such a variation amongst the national parliaments issuing them. The study finds that being a member of the Eurozone and the EU15 has important implications for parliaments issuing reasoned opinions.