Water management is often facing complex problems, which are particularly challenging to address. But while the term 'complexity' has increasingly been used, the concept and its implications for management and governance have often remained unclear. Building on both conceptual and empirical research, this chapter sheds light on complexity in the water field from a management and governance perspective. Analytical concepts of complexity are described and distinguished from related concepts such as 'wicked' and 'uncertain' problems. Further, three types of approaches to address complex problems are discussed, characterized by various understandings of complexity, governance approaches, and emphasis put on inputs (processes) and outputs (results). The chapter provides examples of addressing complex problems, including installing an Integrated Water Resources Management, implementing a Nexus approach to environmental resources and sectors, and addressing poor water quality within the European Water Framework Directive. The chapter concludes on the future role and design of governance research in addressing complex water management problems.