  Principles of the precision cleaning dedicated to ultra-high vacuum
applications are reviewed together with the techniques for the evaluation of
surface cleanliness. Methods to verify the effectiveness of cleaning procedures
are addressed. Examples are presented to illustrate the influence of packaging
and storage on the recontamination of the surface after cleaning. Finally, the
effect of contamination on some relevant surface properties, such as secondary
electron emission and wettability, is presented. This article is an updated and
shortened version of the one previously published for the CAS school on the
vacuum of accelerators 2006.
