A new method was developed by the Image Permanence Institute for monitoring the effects of dynamic environmental conditions on organic materials in order to make it easier to manage the preservation of library and archives collections. This new approach to preservation management applies to the many types of scholarly resources for which research and academic institutions are responsible. The Time-Weighted Preservation Index (TWPI) provides a new way to measure and quantify how temperature and humidity changes affect the preservation quality of storage environments for paper, photographic, and magnetic tape collections, and any other type of organic material. The concept is illustrated with examples and explanations are given on how relatively small changes in storage conditions can result in significant improvements in the useful life of library and archives collections. Plans for the Preservation Environment Monitor are also described. Appendices provide information on the following: forms of deterioration in organic collection materials, deterioration mechanisms and test methods; specific origins of the preservation index (PI) model and how the PI model compares with other published models; and temperature and RH equilibration in TWPI analysis.