This proceedings contains a written record of the Twentieth International Conference on Machine Learning (ICML-2003) held in Washington, D.C. on August 21-24th, 2003. This year’s conference marked several firsts for ICML: For the first time, ICML was collocated with KDD (the International Conference on Knowledge Discovery and Data Mining). We are hopeful that the collocation will encourage further cross-fertilization between the two communities. ICML-2003 and KDD-2003 shared a joint session and an invited speaker. We thank Pedro Domingos, Christos Faloutsos and Ted Senator of KDD-2003 for helping make this possible. For the first time, the conference offered a Student Paper Award program, sponsored by Kluwer and the Machine Learning Journal. We were fortunate to have Robert Holte chair and set the precedent for the Student Paper Award program. In 2002 the International Machine Learning Society (IMLS) was formed as an oversight/advisory committee for the international conference. ICML-2003 was the first conference held under the Society’s guidance.