AbstractKároly Kós, a pioneering master of 20th century Hungarian architecture, spent two years in Istanbul as a fellow of the newly established Hungarian Institute for Science in Constantinople between 1916 and 1917 to pursue research on the architecture of the Ottoman Empire. During this period, he created a whole series of drawings of numerous Byzantine and Ottoman historical buildings and street sections. A volume entitled Istanbul - Urban History and Architecture was published as a summary of his research. However, this historical event and the resulting publication have a far-reaching significance beyond themselves in many ways. Firstly, the aforementioned period was a significant turning point in Ottoman-Turkish architectural history. On the other hand, Kós's work is more than just an analysis of architectural and urban history.This paper aims to provide an insight into the period and the turning point between the late Ottoman and the early Republican era of Turkey's history; the local context of Kós's activities in Istanbul and, at also to analyse the architectural-historical achievements of the Hungarian master's work in the location which he himself described as ‘The City”.