These proceedings contain the papers accepted at the 2007 European PKI Workshop: Theory and Practice (EuroPKI 2007), held in Palma de Mallorca, Spain, during June 28–30, and hosted by the Computer Science Department of the University of Balearic Islands (UIB) with the support of the Balearic Islands Government and the Private Law Department at UIB. This year’s event was the fourth event in the EuroPKI Workshops series. Previous events of the series were held in: Samos, Greece (2004); Kent, UK (2005); and Turin, Italy, (2006). In response to the call for papers, 77 papers were submitted to this year’s workshop, setting a record of the highest number of papers submitted to an EuroPKI event so far and confirming an increased interest in PKI research and in the EuroPKI event. Each paper was reviewed by three members of the Program Committee, and evaluated on the basis of its significance, novelty, technical quality and relevance to the workshop. The paper selection process was very competitive: of the papers submitted, only 21 full papers and 8 short papers were selected for presentation at the workshop and inclusion in this volume.