The term Conservation is often misunderstood, being enterpreted to encompass other terms such as Restoration, Reconstruction and Recreation.
Conservation relates solely to a very specific process whereby an object is maintained in its present condition while precautions are taken to prevent further deterioration. It does not permit the introduction of non original materials, except in certain circumstances.
In the case of stained glass windows it may be necesssary to introduce extra supporting bars to prevent further buckling or sagging. In the event of missing glass it is permissable to replace that glass but under no circumstances should any attempt be made to pass this off as original. In cases like this it wpould be normal to sign and date the replacement pieces. Recovered broken glass is reassembled using the copper foil method. Simply, it is to save what remains of part of our heritage and culture without making it something else.
At all times intervention must be kept to the minimum.

Restoration ... returning a window to a known previous state using its own components, except in the case of lead replacement.

Reconstruction ... returning a window to a known previous state by the introduction of glass replicating that which is too badly damaged to repair.

Recreation .. conjectural reconstruction where a well researched introduction of glass is used to replace missing original pieces.

In all cases minimal intervention and the retention where possible of original glass is of the utmost importance.
Leaded and stained glass windows may be of great historical importance and therefore subject to preservation orders so any work to them could require planning permission or authorisation from an appropriate body.

While stained glass windows, when correctly designed and constructed, are remarkably durable they will normally require attention after c.100 years due to aging.
The most common causes for intervention are perished leadwork, porous cement, buckling or bulging of panels, insufficient supporting bars and loss of paint detail. Damage due to vandalism has become a major concern - unprotected windows being an attractive target for thrown missiles. In this event all fragments of broken glass should be gathered up, labeled and stored for safekeeping. This is imperative for successful repair or reconstruction.

Any conservation work must be carried out in full sympathy with the original artist’s intentions and then only when necessary. Many a valuable window has been  permanently damaged by ill considered, non essential work by incompetent persons.

All custodians of stained glass windows should consider it of the utmost importance to keep photographic records of windows under their care. This is an invaluable resource to any conservator when repairing future damage and will ensure that the resulting work will be as close as possible to the original.