In our digital age, digital media have been being massively produced, easily manipulated, and swiftly transmitted to almost anywhere in the world at anytime. While the great convenience has been appreciated, information assurance has become an urgent and critical issue faced by the digital world. The data hiding, cryptography, and combination of both have been shown not sufficient in many applications. Digital data forensics, which gathers evidence of data composition, origin, and history, is hence called for. Although this new research field is still in its infancy stage, it has started to attract increasing attention from the multimedia-security research community. This seminar addresses the first digit law and its applications to digital forensics. First, the Benford law and generalized Benford law are introduced. Second, their applications to detection of JPEG compression history for given BMP images and detection of double JPEG compression are presented. Finally, the application of first digit law to detection of double MPEG video compression is discussed.