Waterlogged archaeological wood is degraded very slowly compared to wood decay above ground. The special environmental conditions below ground, results in a prolonged decay process that under extremely low oxygen concentration only allow bacterial degradation of wood. The so-called erosion bacteria are described and waterlogged archaeological wood is defined. Soft rot fungi are other microbes that often are found active in more oxygenated aquatic environment. A short historic review on the understanding of waterlogged archaeological wood and the different decay processes is given and exemplified. Knowledge on decay processes is essential for development of conservation methods and in situ preservation of wooden cultural heritage.