Climate change, loss of biodiversity and the growth of an increasingly urban world population are main challenges of this century. With two-thirds of a considerably larger world population predicted to be living in urban areas by 2050, we argue that urban biodiversity, that means the biodiversity within towns and cities, will play an important role by holding the global loss of biodiversity. As a consequence, the Convention on Biological Diversity (CBD) must promote the engagement of cities and local authorities in future. In the first part of this chapter, the efforts of the CBD towards urban biodiversity are analysed from their foundation in 1992 until now. In the second part, the current knowledge of urban ecosystems and their biodiversity is summed up and the importance of urban biodiversity for global biodiversity is highlighted. In the third part, challenges for the future of urban biodiversity are presented. These challenges were addressed to the partners of the CBD during the International Conference 'Urban biodiversity and design - implementing the Convention on Biological Diversity in towns and cities' held in Erfurt in May 2008.