Vacuum breakdowns caused by high voltages and high-power rf are extremely complex phenomena, however significant advances understanding and quantitative modeling are being made. The latest developments in theory, simulation, supporting experimental investigations and application to high-gradient device design will be covered in this workshop. The workshop seeks to bring together the diverse communities such as accelerators, plasma fusion and satellite communication which must contend with vacuum breakdown. This workshop will be preceded by a meeting on the development of high-power X-band rf structures (X-Band Structure) for CLIC.