Book excerpt: "Preventive conservation seeks to minimize needless deterioration, damage, and loss to cultural property. This requires a comprehensive understanding of all hazards that might affect property and all values needing protection from those hazards. The past effects of hazards must be apprehended, the efficacy of current conservation practices must be calculated, and the predictability of future changes must be judged. Risks may manifest gradually or sporadically. In either case, the time scale over which risks to cultural properties must be considered is measured in centuries. Feedback on the consequences of preventive conservation decisions taken is too slow and too confounded to support meaningful experience-based learning. A risk-based approach to decision-making is necessary."