Throughout the twentieth century, research by corporations, universities, and governmental agencies has led to the development of new and improved building materials and systems. Which twentieth-century building materials and systems are historically significant? Why are they significant? Who will make these judgments? And can these materials and systems be preserved? The first section of this paper examines the process of documenting twentieth-century materials and systems; the second discusses the evaluation of twentieth-century buildings, and their materials and systems, within a historic context; and the final section focuses on the importance of retaining and preserving twentieth-century materials and systems.