Motivated by the multiple requirements of data sharing,privacy preserving and knowledge discovery, privacy preserving data mining(PPDM)has become the research hotspot in data mining and information security fields.Two main problem are addressed in PPDM:One is the protection of sensitive raw data;the other is the protection of sensitive knowledge contained in the data,which is also called knowledge hiding in database(KHD). This paper gives a survey on the current KHD techniques.It first introduces the background in which KHD appears. Then it mainly presents the techniques on sensitive association rule hiding and classification rule hiding.Evaluation of KHD methods is discussed after that.Finally,it points out three future research directions of KHD:Design of measure function based on target distance in data modification techniques,inverse frequent set mining in data reconstruction techniques and design of general KHD method based on data sampling.