This publication is aimed at building owners and managers caring for stained glass windows, but it will also be of interest to their professional advisors, and to conservation officers, diocesan advisors and other statutory authorities. It is intended to help readers recognise the symptoms that indicate a decorative window might be suffering from environmental deterioration, and to understand and alleviate the impacts this may be having on the stained glass. The document is not a complete guide to the deterioration of stained glass (the complexity of which demands the expertise of an experienced accredited stained glass conservator), but instead outlines in general terms the process of making decisions about the conservation of stained glass in danger of environmental deterioration. It explains how internal and external environmental conditions can damage decorative windows, and gives pointers for determining whether a window requires specialist attention.