Basic problems encountered in and approaches to conserving geological materials are discussed. Of more than 3500 known minerals, the great majority are metastable, being chiefly affected by variations in environment over long time periods. Some unstable minerals entail special problems. General care of minerals, effects of light, vibration, effects of temperature, effect of relative humidity, and effects of atmospheric pollutants are each discussed under separate headings. These effects are also presented in tabular form for the following categories of minerals: elements and alloys; sulphides and sulphosalts; oxides and hydroxides; halides and oxyhalides; borates; carbonates; nitrates; silicates and aluminosilicates; phosphates; arsenates; vanadates and uranates; chromates; and organic compounds. A separate discussion is provided of problems encountered in the curation of fossils. Separate subsections deal with processing fossil material, storage and handling, consolidation and repair, and relative humidity are provided. Particular stress is placed on knowledge of the properties of the material to be treated, the substances to be used, and the precautions required.