“Green cities” offer a systematic approach to a significant part of the nowadays urban complexity. The concept dovetails in the “healthy city” idea launched by the World Health Organization, but is equally associated with “sustainable” and “smart cities”. During the past decades planning for “green cities” shifted, incorporating new ideas as sustainable development and IT-driven management instruments for smart cities. Contemporary cities continue to face major environmental challenges. Replying to this dynamic context is a main task for cities during the coming decades of the millennium. As most of the (major) cities worldwide are located at the edge of the continents, supporting water-bound activities, they show a significant “blue economy” aspect. This paper reviews the historical context of the science aspects of “green cities” and the related approaches. Four main challenges for livable (coastal) cities today are discussed, taking into account the continuous changes and the almost permanent transition cities face. Climate change effects as sea level rise and extreme weather conditions, affect directly coastal cities; providing enough drinking water is a long standing and increasing problem; ports face particular and specific environmental problems which are in need of a tailored management; and sufficient accessible green areas remain of primary concern for any green city. Cross cutting through these issues are among others mobility and sustainable urban design. These major challenges will necessitate new processes of decision making. Long term planning is essential. 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