A number of topics relating to light and moisture are discussed in the first part of the article. Most of the technical detail has been put in the second part. The subjects discussed are: Chemical change in the museum: Energy of activation, i.e. The energy actually absorbed before any chemical change can take place; Heat energy; Light energy; Extent of the damage. Movement of water in organic material: The closed container at constant temperature; the closed container at varying temperature; temperature container for the Leonardo cartoon; Closed cases and packaging for transport. This section also describes the use of a climatic cabinet (Weyco) to record very small changes in the curvature of a piece of old wood panel painting caused by humidity. Technical Section: Temperature and rate of deterioration; Use of inert gases for the protection of museum objects; The chemistry of embrittlement; Movement and failure.