Data mining represents an emerging technology area of great importance to homeland security. Data mining enables knowledge discovery on databases by identifying patterns that are novel, useful, and actionable. It has proven successful in many domains, such as banking, ecommerce, genomic, investment, telecom, web analysis, link analysis, and security applications. In this chapter, we will survey the main methods and applications of data mining and the information systems recently developed for supporting the mining process. We then overview the key areas of data mining research, in particular, on-line mining of massive data streams, such as those that flow continuously on the Internet and other communication channels. We show that the traditional store-now & mine-later techniques are no longer effective either because of the size of the data stream or because of the real-time response requirement. Thus new fast & light algorithms and suitable systems must be developed for mining data streams. 5.