Late notification of a concentration and failure to notify a concentration constitute a breach of a fundamental aspect of the preventive (ex ante) merger control system in the Republic of Slovenia and the European Union, i.e. the obligation to notify a concentration. As these infringements can result in harmful consequences for effective competition on the market and for consumers, it is essential that infringers are consistently sanctioned. It is important that the imposed sanctions are sufficiently high in order to achieve the deterrent effect. Master thesis comparatively examines the institutes of late notification of concentration and failure to notify a concentration and their consequences in the context of the merger control system in the Republic of Slovenia and the European Union. The main guidelines and circumstances relevant to determine the appropriate level of the fine for the infringements in question are presented through an analysis of the case-law and decisions of competition authorities in those jurisdictions. Master thesis also outlines the concept of obligation to notify a concentration in both systems and proposes certain improvements to it. Given the inherent connection between the obligation to notify a concentration and the obligation to suspend the implementation of the concentration prior to notification (the standstill obligation), the relationship between the two is also presented.