The　exploration　of　the　energetics　of　anaerobic　digestion　systems　can　reveal　how　microorganisms　cooperate　efficiently　for　cell　growth　and　methane　production,　especially　under　low-substrate　conditions.　The　establishment　of　a　thermodynamically　interdependent　partnership,　called　anaerobic　syntrophy,　allows　unfavorable　reactions　to　proceed.　Interspecies　electron　transfer　and　the　concentrations　of　electron　carriers　are　crucial　for　maintaining　this　mutualistic　activity.　This　critical　review　summarizes　the　functional　microorganisms　and　syntroph　partners,　particularly　in　the　metabolic　pathways　and　energy　conservation　of　syntrophs.　The　kinetics　and　thermodynamics　of　propionate　degradation　to　methane,　reversibility　of　the　acetate　oxidation　process,　and　estimation　of　microbial　growth　are　summarized.　The　various　routes　of　interspecies　electron　transfer,　reverse　electron　transfer,　and　Poly-beta-hydroxyalkanoate　formation　in　the　syntrophic　community　are　also　reviewed.　Finally,　promising　and　critical　directions　of　future　research　are　proposed.　Fundamental　insight　in　the　activities　and　interactions　involved　in　AD　systems　could　serve　as　a　guidance　for　engineered　systems　optimization　and　upgrade.