The importance of the 1980 United Nations Convention on Contracts for the International Sale of Goods (‘CISG'') is widely recognised all over the world. The CISG''s important role in international trade is closely connected with the idea its drafters had in mind while working on it, which was to produce a uniform body of law. The objective was to remove existing legal barriers in this field of law and to promote its development. However, due to the compromise-driven drafting process and the complexity of the law itself, inevitable gaps – outside as well as within the scope of the CISG – and symptoms of aging are visible. It is tempting to directly resort to domestic law in order to fill such gaps, but also when ambiguities or uncertainties arise. Recourse to domestic law is however not compatible with the CISG''s goal to achieve uniformity. Domestic law has thus to remain the last exit. A uniform approach for filling gaps, which minimises the use of domestic law, is decisive. This monograph shall sensitise readers accordingly and help promoting a uniform gap- filling approach.