Dilution as a legal doctrine has been around since it was first mentioned by the German Landgericht Elberfeld Court in the Odol case in 1924 and yet it creates a lot of confusion among lawyers and judges. Approaches taken by national courts vary significantly despite the existence of the ‘hard’, or binding law, and ‘soft law’ (quasi- legal instruments which do not have any legally binding force) addressing dilution of a trademark. The purpose of this thesis is to identify what constitutes dilution in the EU and US jurisdictions, to provide a comprehensive analysis of all the elements that are considered by courts when testing for dilution and to indicate the defenses against dilution claims in a form of legal use. The thesis is split into three main sections. Chapter I gives a brief overview of the legal ground for protection of marks with renown in the EU and US, explicitly determining a content of protection and a standard for harm that is sufficient to invoke dilution laws. Chapter II examines the international and national legal provisions to establish the scope of renown of the mark to enjoy the dilution protection, mapping out in particular differences between the ‘famous marks’, ‘marks with repute’ and ‘well-known marks’, indicating their correlation, and outlining the evidence to prove renown. Chapter III analyzes all the elements that are required to succeed on a dilution claim, namely: the illegal use of the similar mark and the harm caused by such use in forms of blurring, tarnishment and free-riding, and provides an insight on legal use of the mark with renown, which could serve as a defense for the later user of this mark.