This volume contains the papers presented at the 20th International Conference onAlgorithmicLearningTheory(ALT2009),whichwasheldinPorto,Portugal, October 3-5, 2009. The conference was co-located with the 12th
International ConferenceonDiscoveryScience (DS 2009). The technicalprogramof ALT 2009 contained 26 papers selected from 60 submissions, and 5 invited talks. The - vited talks were presented during the joint sessions of both
conferences. ALT 2009 was the 20th in the ALT conference series, established in Japan in 1990. The series Analogical and Inductive Inference is a predecessor of this series: it was held in 1986, 1989 and 1992, co-located with
ALT in 1994, and subsequently mergedwith ALT. ALT maintains its strong connections to Japan, but has also been held in other countries, such as Australia, Germany, Hungary, Italy, Singapore, Spain, and the USA. The ALT series
is supervised by its Ste- ingCommittee:NaokiAbe(IBMThomasJ. WatsonResearchCenter,Yorktown, USA), Shai Ben-David (University of Waterloo, Canada), Phil Long (Google, Mountain View, USA), Gábor Lugosi (Pompeu Fabra University,
Barcelona, Spain), Akira Maruoka (Ishinomaki Senshu University, Japan), Takeshi Shi- hara (Kyushu Institute of Technology, Iizuka, Japan), Frank Stephan (National University of Singapore, Republic of Singapore), Einoshin
Suzuki (Kyushu U- versity, Fukuoka, Japan), Eiji Takimoto (Kyushu University, Fukuoka, Japan), GyörgyTurán (University of Illinois at Chicago, USA, and University of Szeged, Hungary), Osamu Watanabe (Tokyo Institute of
Technology, Japan), Thomas Zeugmann (Chair, Hokkaido University, Japan), and Sandra Zilles (Publicity Chair, University of Regina, Canada). The ALT web pages have been set up (together with Frank Balbach and Jan Poland) and
are maintained by Thomas Zeugmann.