Technical Abstract: Armillaria root disease affects fruit and nut crops, timber trees and ornamentals in boreal, temperate and tropical regions of the world. The causal pathogens are members of the genus Armillaria (Basidiomycota, Physalacriaceae). This review summarizes the state of the knowledge, highlights recent advances in the research, and points out areas for further research. Armillaria species are sexual fungi with heteromictic (heterothallic) and homoheteromictic (secondary homothallic) reproductive cycles. The genus has had a confused and controversial taxonomic history. The discovery of biological species within A. mellea resulted in the extensive use of this concept in the taxonomy of Armillaria. As plant parasites, Armillaria species are necrotrophs. As saprophytes, they are white-rotters, and are, thus, capable of lignin degradation. A host exhibits above-ground symptoms (stunted shoots, dwarfed leaves, wilting) when diagnostic mycelial fans of the pathogen encircle the root collar, thereby girdling the trunk and disrupting root uptake of minerals and water. Recent research on post-infection controls (cultural, biological, chemical) has revealed promising alternatives to the former pre-plant eradication attempts with soil fumigants, which are now being regulated more heavily or are banned outright, due to their non-target effects on the environment. With new study tools that allow for genetic manipulation of the pathogen and evaluation of the host response to infection, development of resistant rootstocks is the next stage of research in control of Armillaria root disease.