This publication contains the papers presented at the 15th European Conference on Soil Mechanics and Geotechnical Engineering (ECSMGE), held in Athens, Greece. Considerable progress has been made in recent decades in understanding the engineering behavior of those hard soils and weak rocks that clearly fall into either the field of soil or of rock mechanics, and there have been important developments in design and construction methods to cope with them. Progress would be even more desirable, however, for those materials which fall into the ‘grey’ area between soils and rocks. They present particular challenges due to their diversity, the difficulties and problems arising in their identification and classification, their sampling and testing and in the establishment of suitable models to adequately describe their behavior. The publication aims to provide an updated overview of the existing worldwide knowledge of the geological features, engineering properties and behavior of such hard soils and weak rocks, with particular reference to the design and construction methods and problems associated with these materials. Part 4 was published post-conference and includes Conference Reports.