This thesis aims to examine the economic effect in terms of real GDP at constant 2017 national prices (in mil. 2017 US$) of being a member in the European Union in the case of Sweden. The method used to answer the research question is mainly by the Synthetic Control Method. Difference-in-difference estimations were also conducted as a complement to compare the results between the two methods. Previous literature in the subject has for the most part received results indicating that a membership in the European Union contributes to an increase in certain economic growth aspects for the member countries. The economic theories about common markets points at that a common market can be both advantageous but also disadvantageous depending on the setting. Theories more closely related to the European Union present rather ambiguous answers to certain economic effects of a membership but in some cases, it seems to be possible to assume some positive economic effect from being a European Union-member. The results in this paper seem to be robust in the way that the effect on the real GDP at constant 2017 national prices (in mil. 2017 US$) in Sweden from its membership in the European Union is negative in all estimations that were made. However, none of the results were statistically significant so there is not enough evidence to say that the Swedish real GDP at constant 2017 national prices (in mil. 2017 US$) would have been different to the factual outcome if Sweden did not join the European Union in 1995.