Dampness on the inside surfaces of dwellings is a frequent source of complaint. It may be due to rising damp, rain penetration or a plumbing defect; or it may be due to condensation. Condensation and mould growth are widespread problems in all housing sectors but especially so in tenanted accommodation. In many cases it may be difficult to identify the underlying cause; this can often be complicated by social issues. Mild cases will often yield to simple changes in the heating and ventilation regime in the dwelling or to cosmetic treatments of redecoration, perhaps with fungicidal paint. In more severe cases fungicidal treatments may be little more than a useful holding operation if major rehabilitation is not possible for some time. More severe cases will usually require improvements to thermal insulation, greater heat inputs and a reappraisal of ventilation (either natural or mechanical) of the actual dwelling. Mould growth varies in severity, causing 'inconvenience', 'discomfort' or 'acute distress'. This digest considers the circumstances that lead to surface condensation and mould growth and suggests ways of reducing their incidence in dwellings of traditional construction.