General principles have played a signifi cant role in the development of EU law from the very beginning; they were needed to fill gaps in its fragmentary structure. The difficulties of a general definition, of their ascertainment and of establishing their rank in the law of the Union have been outlined above; what could be assessed is a distinction between structural principles arising from the nature of the Union’s legal order as a conglomerate of national law and common rules, and external principles derived from districts of the law outside EU law, in particular from comparative law. While defying an overarching description, general principles have been acknowledged in numerous specific instances. Parts IV to VI contain a non-exhaustive list of specific principles which are grouped, according to their aim and focus, in three clusters: principles which are constitutive for any legal community in part IV , principles of the market economy in part V , and principles of solidarity in part VI .