This book discusses the function, services, importance and threats of natural and constructed wetlands. The first chapter presents one of the largest wetlands which is located in the West Siberian Lowland. This chapter presents the Lowland’s geological history and its impact on the contemporary forms of wetlands in the form of peatlands, rivers and their floodplains, lakes, river deltas, estuaries, and swamps. A large part of the chapter is devoted to the inventory of these areas, their environmental functions, and their threats. Chapter Two, which is also related to the wetlands of the West Siberian Lowland, presents the impact of peatlands and swamps on the seasonal physicochemical composition of waters in rivers that are fed from peatlands and swamps. The third chapter concerns the threats to peatlands of the West Siberian Lowland, which are a consequence of oil extraction. The chapter also describes how the composition of peat changes due to this type of pollution and how it affects the vegetation. Chapter Four presents an extensive review of the ecology and geomorphology of the Danube River delta, which is one of the best preserved and more complex deltas in Europe in terms of evolution, morphology, landscapes, and biodiversity. Chapter Five presents the inventory and threats to wetlands in Oceania and South Asian countries; special attention is given to the type of threats to the wetlands in each country. Chapter Six discusses the current state of wetlands and their importance in Central Europe, using Hungary as an example. Chapter Seven focuses on the International Convention on Wetlands (Ramsar Convention) and presents the Convention’s history, development, and its main bodies. The chapter also discusses the evaluation of its protection measures based on the example of the Polish environmental conservation system. Chapter Eight presents the multiservice and functions of constructed wetlands in order to boost this infrequently used technology and highlight the diversity of its applications. The last chapter presents the evolution of constructed wetlands used for wastewater treatment, which begins with irrigation fields and finishes with a modern solution as improved wetland systems are combined with water reuse.