In this book the authors review the structure of an ecosystem and of its epigean and hypogean parts, defining uniquely some physical, chemical, and climatic constituents that are fundamental for understanding subterranean communities. They discuss the factors that led, for decades, to an ill conceived emphasis on the importance of human-size caves as the natural seat of “cave-dwellers”, paying special attention in defining the subterranean environment in the Euro-Mediterranean area and of its limits, with some of its different components (fissure network of the bedrock, MSS, cave and subterranean hygropetric environment), compared to the endogean environment that is characteristic of the soil, by restricting the use of the term subterranean fauna only to the hypogean environment, with the exception of the endogean environment. Similarly, the faunas associated with the subterranean environment are more clearly defined, as well as the relationship between the fauna and the fissure network of the bedrock, and the influence of bedrock and tree cover on MSS formation and colonization. The authors describe in detail the methodology and techniques used in their multi-annual research on the subterranean fauna in Greece, that allowed them to discover hundreds of new species to science.