Apart from crises such as floods or fires, which strike without warning, the signs of potential damage to works on paper can usually be identified by an alert professional. Early detection of these signs requires that staff be keenly observant of the collections, their individual housing (mats, folders, boxes), and the facility in which they are stored and displayed; staff must also be sufficiently knowledgeable to recognize when conservation or other corrective measures are in order. The major damage to works on paper is caused by photooxidation, acid migration, biodeterioration, mechanical injury, and improper framing and storage. For each of these categories there are warnings that signal a potential problem and visible signs that point to existing damage. Either circumstance may require conservation treatment, but by taking preventive steps many problems can be entirely averted or reduced in severity.