Archaeological wood and wood of historic value from different environments were examined to determine the type of degradation present, extent of deterioration and to characterize the current condition of the wood. Micromorphological and ultrastructural investigation showed diverse forms of biological as well as non-biological deterioration. Wood from a cabinet in the Thomas A. Edison Laboratory Museum, Fort Myers, Florida (40-60 years old) appeared soft and shredded. The unusual deterioration resulted from a chemical spill of caustic substance(s) that gradually delignified the wood. Middle lamellae were removed from the woody cell walls leaving detached secondary wall layers. Samples of furniture and tomb structure from Tumulus MM, Gordion, Turkey (700 BC), considered to be the tomb of King Midas, had extensive soft rot. Extensive cavity formation was observed throughout the S2 layers of secondary walls. Egyptian samples (3000-1000 BC) were attacked primarily by brown rot fungi. Localized areas of severely degraded wood contained cells that appeared porous and lacked structural intergrity. The lignified residue that remained fragmented into dust-like particles. An unusual form of soft rot with cavities within the S1 layer of the secondary wall also was found in one Egyptian object. Ancient buried, waterlogged wood (10 000-4000 BC) was degraded by tunneling and erosion forms of bacteria. Minute cavities and a diffuse attack of the cell wall occurred. These ultrastructural results provide information concerning the current condition of the wood and also a chronology of past degradative processes that are essential to ascertain the most appropriate methods of conservation.