Most ancient bronzes have been corroded and present many problems to the conservator. They must be treated not only as works of art but as documents. The form of corrosion in which the original surface is preserved within the corrosion layers is described. For every object more than one method of conservation has to be considered. Problems of cleaning, protection against corrosion, repair, examination and documentation are presented from the point of view of the laboratory. Special points of mention are cleaning by electrolysis and the importance of ensuring that samples for analysis are representative.