The rumen microbial community transforms feed into nutrients that the ruminant host animal absorbs and utilises for maintenance, growth and fattening, reproduction and lactation. Volatile fatty acids produced by microbial fermentation in the rumen are the main source of energy, glucose and fat for ruminants, and the ATP generated drives microbial growth, which provides amino acids to the host. Methane formed in the rumen is a greenhouse gas and an energy loss for ruminants. Whether inhibiting methanogenesis can benefit animal productivity depends on various factors that need to be better understood. The efficiency of microbial growth is affected by maintenance energy, cell composition including glycogen accumulation, energy spilling, and cell lysis. Research is needed on maximising the energy output of fermentation, controlling the profile of volatile fatty acids, and increasing the efficiency of microbial growth.