The objective of this paper is to present some ambiguities relating to engineering properties of shales and its classification for design consideration. Because of its unpredictable behavior, shales are the most difficult rock to classifying, sampling and obtaining reliable test data for engineering purposes. It is the most problematic weak rocks that cause many problems in the field before and after construction or excavation. In this paper, engineering properties of shales and its classification are briefly discussed. Both in the field and laboratory, common engineering problems and some useful aspects of the rock are evaluated in terms of design consideration. Consequently, origin of shale and related rocks, its classification and properties are discussed and common testing procedures are evaluated for engineering design purposes.