The topic of cleaning covers a large technical area of conservation that has been subject to periods of considerable controversy, which has constrained progress. The complexity of the technical debate on cleaning has limited useful discussion outside the conservation profession. To understand the subject in its entirety, many interpretive skills are required for a multidisciplinary approach. Scientific research, as demonstrated by the acrylics cleaning project, can provide conservators with better knowledge of materials, new ideas, and tools. A history of artist varnishing and conservation cleaning practice is limited by the paucity of literature and material evidence. Trends must be inferred from more recent practice. Developments in solvent, aqueous, and mechanical methods for varnish removal and for cleaning unvarnished surfaces are briefly discussed. Examination and analysis of paintings and materials are keys to understanding artists’ methods more precisely and to interpreting their intentions better.