We shall not be dealing on this occasion with surface cleaning of varnishes. This is normally a straightforward exercise unless the varnish is particularly degraded, in which case the conservator has to consider the problems of varnish removal. Our interest is in the more demanding exercise of protecting unvarnished paint-films from particulates and aerosols in their environment. We hope to identify the main problems and present some solutions for consideration. This certainly will not be the last word on dirt removal but I trust it will stimulate some insights and provide a basis for further developments. To do so we need to consider the mechanism of deposition, the visual effects, and also the prevention and removal of extensive layers of dirt. The adhesion of dirt to paint is a fascinating subject as yet only poorly understood in the most general terms, but an understanding is crucial to any assessment of cleaning methods. Further advances in cleaning methods may come about by trial and error, but we hope that by taking stock of current knowledge we may help better define future studies and at least disseminate our experiences more widely.