The M-w=7.4 earthquake that affected the northwestern part of Turkey oil August 17, 1999, and in particular the gulf of Izmit, had dramatic consequences also as regards tsunami generation. The main cause of the earthquake was a dextral strike-slip rupture that took place along different segments of the western part of the North Anatolian Fault (WNAF). The rupture process involved not only a number of distinct strike-slip fault segments, but also dip-slip ancillary faults, connecting the main transcurrent segments. The general picture was further complicated by the occurrence of subsidence and liquefaction phenomena, especially along the coasts of the Izmit bay and in the Sapanca Lake. Tsunami effects were observed and measured during post-event surveys in several places along both the northern and the southern coasts of the bay. The run-up heights in most places were reported to lie in the interval 1-3 m: but in the small town of Degirmendere, where a local slump occurred carrying underwater buildings and gardens of the waterfront sector, eyewitnesses reported water waves higher than 15 m.