Systemic integration has made its way into the parlance and practice of international scholars and adjudicators, particularly as a tool against the fragmentation of international law. The chapter analyses it as a particular instance of what Foucault termed ‘problematization’, that is, ‘how and why certain things (behavior, phenomena, processes) became a problem’. In fact, while the issue of fragmentation is not a recent phenomenon, only in the late 1990s did it receive special attention. Why was it so? The question of what drove most international lawyers to present fragmentation as one of the most pressing issues of the time, and the International Law Commission to issue a report on fragmentation and international law, prepared by Martti Koskenniemi in 2006, is examined. The chapter argues that the wide acceptance of the systemic integration principle could be explained by international lawyers’ continuous quest for unity, uniformity, and coherence.

Customary international law is one of the formal sources of international law and plays a pivotal role in the existence and functioning of the international legal system. Although for a rule of CIL to emerge a widespread, representative, constant and virtually uniform state practice is required, accompanied by the requisite opinio juris, that does not necessarily mean that CIL is a slow and archaic process, which has been overcome by extensive treaty-making. On the contrary, CIL remains a vital element in the corpus of international law that is open to refinement, clarification, development and evolution. This process does not happen only through the classical emergence and/or subsequent modification of the rule, but also and perhaps most importantly through the process of interpretation. This chapter demonstrates this by showing that CIL interpretation is neither problematic from a theoretical perspective, nor is it the only example of interpretation of unwritten rules. This is further reinforced by jurisprudence taken from both the domestic and the international legal system, although such interpretative exercises are not without their limits. What emerges from this analysis is that CIL interpretation, as Sur has beautifully noted, is of a negentropic nature that constantly nourishes and updates CIL.

This chapter lays out a novel framework for conceptualising the water-relevant binding and non-binding instruments of the UNECE environmental conventions as one common normative regional environmental regime – an original contribution. In exploring the idea of a single regime, this chapter seeks to overcome a strictly positivist view of international law and understand the relevance of overlapping and/or non-uniform state membership of the UNECE legal instruments. It explores regionalism and regional approaches to international law and examines the relationship of the UNECE regime to international law and other international institutions. It sets out a framework for determining the UNECE regime’s relationship to general international law and other international water treaties – asking whether the regime lex specialis – a theme returned to throughout the remainder of the book. This chapter sets out a framework for exploring the making, implementation and enforcement of international law in the UNECE regime, which is employed throughout the research. This frame contributes to understanding around systemic integration, mutually supportive interpretation and cross-fertilisation in international environmental law and international law relevant to transboundary freshwater ecosystems.