The need for protecting or rehabilitating aquatic habitats in urban areas and the need for incorporating such management actions into urban water management have been briefly introduced in Chapter 1. The main purpose of this chapter is to further develop the urban aquatic habitats description, with respect to their basic characteristics and functioning, and set the stage for further discussions of their management in the following chapters. Understanding the habitat characteristics that affect aquatic biological communities, as weil as the functioning of the aquatic ecosystem, helps to set realistic objectives for the protection or rehabilitation of its ecological status or potential and set implications for management actions. Various aspects of aquatic habitat protection and rehabilitation and their integration with urban water management with respect ta strategies, planning and technical measures are addressed later on in this book.