We are pleased to present the proceedings of the 12th International Conference on Discovery Science (DS 2009), held in Porto, Portugal, October 3-5, 2009. DS 2009 was collocated with ALT 2009, the 20th International Conference on AlgorithmicLearningTheory, continuingthesuccessfulDSconferenceseries. DS 2009 provided an open forum for intensive discussions and the exchange of new ideas among researchers working in the area of discovery science. The scope of the conference included the development and analysis of methods for automatic scienti?c knowledge discovery, machine learning, intelligent...