This thesis examines the twentieth century architecture from the perspective of conservation. The criteria of conservation have changed as the idea of conserving a single monument has progressed into the acceptance of the need to conserve different cultural properties and the field has been enriched with new notions, such as the twentieth century architectural heritage. The main concern in this thesis is to present these current debates about and developments in the conservation of the twentieth century architecture in the world and in Turkey. Such a study initially entails to deal with the basic issues of conservation, the twentieth century architecture in the world and in Turkey and its conservation, and to form a detailed documentation of registered twentieth century buildings. With reference to the information gathered from this study and by examining the registration decisions of buildings the aim is to analyse the practice of the conservation of the twentieth century architecture in Ankara as an exemplary case of the current situation of the field in these terms in Turkey.