Restoration is a process that attempts to return cultural heritage to some previous state that the restorer imagines was the "original". This was commonly done in the past. However, in the late 20th century a separate concept of conservation-restoration was developed that is more concerned with preserving the work of art for the future, and less with making it look pristine. Restoration is controversial, since it often involves some irreversible change to the original material of the artwork with the goal of making it "look good." The attitude that has developed in recent years with the development of conservation is to attempt to make all restoration reversible.