This book places the biodiversity of soil invertebrates in an evolutionary framework, showing the various adaptations to the soil environment. Species radiations are discussed for each of the terrestrializations leading to a lineage of soil invertebrates. Phylogenetic analyses, now becoming available for most of these groups thanks to DNA sequencing, show monophyly of complete soil-living groups in some cases and multiple independent soil adaptations in others. The book includes many classical methods of soil animal enumeration and community surveys developed in the 1970s, which can now be supplemented by physiological and molecular study techniques.