The master thesis initially contains the subject and the object of research, and it primarily refers to the change of currency and the operation of SMEs in the Internal market. From this issue, it follows that the monetary integration of individual members of the European Union does and results in changes for their small and medium sized businesses. The theoretical framework defined what small and medium-sized entrepreneurship is, how it behaves under conditions of monetary integration, and which legislative frameworks the country adapts to, and which directly or indirectly affects entrepreneurship within that country. Entrepreneurship in Slovenia, Slovakia, Lithuania and Croatia was analyzed. The analysis found that entry into the eurozone alone is not enough to stimulate and improve the business conditions of SMEs. The analysis of Slovenia and Lithuania found that the transition year had a positive effect on SMEs in that the number of employees, the number of newly opened companies and the share of added value increased significantly. The analysis of Slovakia revealed different results, rather a decline in employment and a incline in the number of companies. It follows how the monetary integration of the new EU Member States works and results in changes for small and medium-sized businesses. By entering the euro area, Croatian SMEs can be forecasted to have a positive further effect, considering the upward growth in employment and the number of companies, immediately before the introduction of the euro, which confirms the estimates of the European Commission.