This book constitutes the refereed proceedings of the 9th European Conference on Principles and Practice of Knowledge Discovery in Databases, PKDD 2005, held in Porto, Portugal, in October 2005, jointly with ECML 2005.

The 35 revised full papers and 35 revised short papers presented together with abstracts of 6 invited talks were carefully reviewed and selected from 220 papers submitted to PKDD and 30 papers submitted to both, PKDD and ECML. The papers present a wealth of new results in knowledge discovery in databases and address all current issues in the area.