Database security is one of the classical topics in the research of information system security. Databases are no longer stand-alone systems that are only accessible to internal users of organizations. Instead, allowing selective access from different security domains has become a must for many business practices. Many of the assumptions and problems in traditional databases need to be revisited and readdressed in decentralized environments. Further, the Internet and the Web offer means for collecting and sharing data with unprecedented flexibility and convenience. New data services are emerging every day, which also bring new challenges to protect data security. We have witnessed many exciting research works toward identifying and addressing such new challenges.