This chapter discusses a systemic understanding of soil and also explores soil fauna. Soil fauna is commonly subdivided into three broad groups according to the size of organisms: microfauna, mesofauna and macrofauna. The wildlife biodiversity is extremely high in soils and the chapter discusses the quantification methods of biodiversity. Then, the chapter describes the effects of the abiotic environment and discusses the biotic interactions in the soil. Further, it presents the state of knowledge about diversity-functioning relationships, which will lead the readers to question the use of wildlife as an indicator of soil quality. Numerous studies have shown that soil fauna can be used as a bioindicator of soil pollution. In order to assess the biological quality of soil, or at least the loss of soil biodiversity, a number of indicators are essential. The chapter further tabulates these indicators.