1 Introduction.- 2 Fundamental Aspects.- Wood Anatomy.- Structure of the Lignified Cell Wall.- Middle Lamella.- Primary Wall.- Secondary Wall.- New Information on the Structure of the S2 Layer.- Mechanical Model for Wood.- Types of Wood Decay.- Brown Rot.- White Rots: the Whole Spectrum of Diversity.- White Rot.- White Rot: Selective Delignification.- White Rot: Simultaneous Rot.- Soft Rot.- Soft Rot in Living Trees Caused by an Ascomycete.- Basidiomycetes as Facultative Soft-Rotters.- 3 Examination, Importance and Mechanical Effects of Wood-Decay Fungi in the Living Trees.- Fungus Identification.- Basidiomycetes.- Ascomycetes.- Differentiation and Asexual Reproduction of Mycelium.- Classification of Wood-Decay Fungi According to Their Main Occurrence in the Tree.- Colonization Strategies of Stem-Decaying Fungi.- Colonization Strategies of Root-Decaying Fungi.- Number of Fungal Fruit Bodies and Their Predictive Value.- Fungus Species Which Invade the Stemwood as Well as the Roots.- Root Fungi Found Mainly Only in the Roots.- Wood-Decay Patterns Observable in the Roots.- Factors Influencing Wood Degradation.- Symptoms of Damage by Fungal Pathogens.- Symptoms of Damage by Stem and Root-Decaying Fungi.- Fungus-Host Combinations.- Fomes fomentarius.- Distribution and Hosts.- Fruit Bodies.- Possibilities of Misidentification.- Useful Information.- Appearance of the Decay.- Pattern of Wood Decay by Fomes fomentarius (on Beech and Oak).- Inonotus hispidus.- Distribution and Hosts.- Fruit Bodies.- Useful Information.- Appearance of the Decay.- Pattern of Wood Decay by Inonotus hispidus (on London Plane and Ash).- Laetiporus sulphureus.- Distribution and Hosts.- Fruit Bodies.- Useful Information.- Appearance of the Decay.- Pattern of Wood Decay by Laetiporus sulphureus (on Robinia and Oak).- Fistulina hepatica.- Distribution and Hosts.- Fruit Bodies.- Useful Information.- Appearance of the Decay.- Pattern of Wood Decay by Fistulina hepatica (on Oak).- Ustulina deusta.- DistrlĂ-