Anaerobic digestion is a widely applied process to recover energy and nutrients duringthe stabilization of organic waste or to produce a renewable energy carrier in form of biogasfrom various organic materials. The process application ranges from simple householdscale to complex industrial scale systems to produce a methane-rich biogas that can beutilized for heating and cooking, electricity and heat provision or a replacement of naturalgas.The anaerobic digestion process is a natural process conducted by various microbialgroups in oxygen-free environments. The consecutive process phases during thedecomposition of organic matter to biogas are characterized by so-called syntrophicrelationships, where all involved microbial groups depend on successful collaborationalong the various process phases. However, not all microbial groups share the samerequirements to their environments imposing compromises on some groups to maintainphysiological activity of the total system. Additionally, some microbial groups are moresensitive to inhibitory compounds that might be introduced into the process with theorganic material or formed during the degradation. The maintenance of suitable conditionsfor all involved microbial groups is the major challenge especially during anaerobicdigestion at industrial scale, where consistent treatment or production efficiency isrequired. The following chapter provides the basis to understand the microbialinterrelationships and process conditions to exploit anaerobic digestion as an economicallyand ecologically beneficial technology.