© 2012 Taylor & Francis Group, London.The uncontrolled increase in population of Istanbul is causing many problems in civil life. Many areas are being opened to settlement without sufficient preliminary engineering investigations because of the increasing demand for new infrastructure. In the western part of Istanbul, much unplanned construction, of various types and dimensions, is undertaken on relatively weak Tertiary units. These bedrock units outcrop mostly in the southern and western part of Istanbul and have many instability, swelling and shrinkage problems. Besides, these problematic ground conditions contributed to the collapse, tilting and rotating of many buildings and led to many casualties during 1999 Izmit Earthquake. In this study, the geological, hydrogeological and geotechnical properties of the Tertiary units of Istanbul were investigated for the assessments of mass movements in the western part of the city. According to our investigations, it should be noted that small-scale buried faults, lithologies and the hydrogeological properties of the units are important.