Strong-motion networks installed in the Hagia Sophia Museum and in the Süleymaniye Mosque in Istanbul, Turkey, recorded a series of events during the 17 August 1999, Kocaeli (Mw 7.4) and 12 November 1999, Düzce, Turkey (Mw 7.2) earthquakes and during the following aftershock activity. The time-domain and frequency-domain analysis of data presented in this article yielded the following results: On the basis of the pre-1999 recordings in Hagia Sophia it was observed that the modal frequencies tend to change with amplitude and duration of shaking. This observation is supported by the recent data as well. Local problems were noted in Hagia Sophia in the southwest main pier and possibly in Süleymaniye in the west main arch. Significant vertical vibrations at the crowns of the east and west main arches in Hagia Sophia are probably indicating locations where most of the damage is to be expected during a major earthquake close to Istanbul. A postearthquake, quick ambient vibration survey conducted in Hagia Sophia showed a frequency drop in the first two modal vibrational frequencies on the order of 4%–5%, indicating the need for a comprehensive ambient vibration study. Modal shapes and first-order estimates of modal damping of the two structures have also been assessed.