This volume contains the papers presented at the Second International Conf- ence on Discovery Science (DS'99), held in Tokyo, Japan, December 6-8, 1999. The conference was colocated with the Tenth International Conference on Al- rithmic Learning Theory (ALT'99). This conference was organized as part of the activities of the Discovery S- ence Project sponsored by Grant-in-Aid for Scienti c Research on Priority Area fromthe MinistryofEducation,Science,SportsandCulture (MESSC)ofJapan. This is a three-year project starting from 1998 that aims to (1) develop new methods for knowledge discovery, (2) install network environments for kno- edge discovery, and (3) establish Discovery Science as a new area of computer science. The aim of this conference is to provide an open forum for intensive disc- sions and interchange of new information among researchers working in the new area of Discovery Science. Topics of interest within the scope of this conference include, but are not limited to, the following areas: Logic for/of knowledge discovery, knowledge d- coverybyinferences,knowledgediscoverybylearningalgorithms,knowledged- coverybyheuristicsearch,scienti cdiscovery,knowledgediscoveryindatabases, data mining, knowledge discovery in network environments, inductive logic p- gramming, abductive reasoning,machine learning,constructive programming as discovery,intelligentnetworkagents,knowledgediscoveryfromunstructuredand multimedia data, statistical methods for knowledge discovery, data and kno- edge visualization, knowledge discovery and human interaction, and human f- tors in knowledge discovery. The DS'99 program committee selected 26 papers and 25 posters/demos from 74 submissions. Papers were selected according to their relevance to the conference, accuracy, signi cance, originality, and presentation quality.