This thesis has been commissioned by the Delft University of Technology for the course AR2A011 Architectural History Thesis. This thesis focusses on the city of Istanbul. Istanbul enjoyed a unique status as the capital city of two great civilisations: the Byzantine Empire and the Ottoman Empire. Over the centuries, the city absorbed and reflected the cultural heritage of its rulers; Eastern Orthodox Christianity followed by Islamic Ottoman civilisation. Its metamorphosis from “Roma Nova" to the "sacred city of Islam" produced unparalleled configurations in its urban form. As a result of this, Istanbul nowadays faces great fragmentation of its urban plan. In this urban plan different heritage buildings and historic urban plans collide. The directory of this thesis will be to provide a solution on how in 21st century Istanbul can deal with its fragmented urban plan, while also preserving its complex heritage. Therefore the main question states: How should the fragmentation of Istanbuls urban plan be dealt with in the future, while preserving its heritage both ancient, historic and modern? In order to conclude on this several subquestion have been drawn up. These state: I. What ancient heritage, cultural and architectural, are still visible in Istanbul? II. How did the nineteenth century modernization movement in Istanbul viewed and dealt with the heritage? III. What are the main principles to deal with heritage? Which of these principles are applicable to the present day urban plan of Istanbul? Opening with an architectural survey of The Ancient, Byzantine and Ottoman city the complex heritage of Istanbul is explained. This is followed with the description of the late nineteenth modernization movement, essentially being a longing to the west, which contributed to the fragmented urban plan by implementing an architectural plurality in Istanbul. Finally in order to make a proposition, a framework by Janssen et al. is introduced that shows three approaches on how to deal or incorporate heritage in spatial design. Evidently, Istanbuls heritage both ancient, historic and modern is very complex. However the same heritage also created issues as mentioned above. Istanbuls urban plan needs to be revised in order to exterminate said issues. Therefore a mixed-mode model of dealing with heritage needs to be applied in order to preserve as much of the value of this heritage. By using heritage as a tool in spatial design, not only will the heritage be preserved but also eliminates the obvious roadblocks of progress that can be a result of preservation. Furthermore, the fragmentation of the urban plan can be brought to a minimum if the spatial vision on the future will see Istanbul as a whole.