As explained, the purpose of Part 2 is to identify the national nucleus of the principle of legality, i.e. an approximation of this principle that could inspire the construction of a European legality principle. Therefore, after analysing the theoretical justifications of the legality principle, an analysis of the legality principle's national application in three national systems (German, English and Dutch) shall be presented.

This Chapter shall focus on the modus operandi of the principle in national law. It shall observe how this principle is understood in national law both in theory and practice and the challenges faced by domestic legal systems. The aim is to provide the features of this principle in national systems and the problems that exist. The goal is not to devise solutions or a complete theoretical narrative of the legality principle; this will be the focus of the next Chapter (Chapter V).

The present Chapter starts with comparing the ways national systems interpret legal theory and the nature of the legality principle. As will be shown there is a clear difference between the common and continental systems, as the legality principle is the result of each jurisdiction's unique historical development and legal culture. The Chapter continues with an analysis of the ECtHR approach on this principle and its influence on the national legal systems. This Chapter ends with a shift from the ‘law in the books’ to the ‘law in practice’ and a problem-based illustration of the current application of the doctrine in national law shall be presented. What will be shown is that the application of this principle presents common problems to all three jurisdictions, despite their doctrinal and historical differences. While common and continental law systems have had diametrically opposed starting points, the traditions are converging and increasingly similar challenges exist.

Before starting with the illustration of the modus operandi of this principle in a comparative manner, certain rules and disclaimers are required. First of all, it should be kept in mind that the legality principle is a difficult concept to compare as it is influenced by variables distinctive to each jurisdiction.