Over the past three decades, Integrated Water Resources Management (IWRM) has evolved into one of the leading water management paradigms. Revisiting the starting points and the development of the IWRM concept, this chapter critically analyzes the rationales and the major elements to be considered in the framework of IWRM. IWRM is then related to other recently emerging concepts such as adaptive water management and the Resource Nexus. Even though IWRM has been formally adopted almost worldwide for almost two decades, its implementation remains a challenge for many countries. IWRM also became a major research topic in water sciences and beyond, calling for a reflection of its role and impact. Based on theoretical and empirical analyses of contemporary IWRM research, this chapter provides best practice examples of science based implementation and synthesizes the lessons learnt.