The livestock sector has grown to a massive size, with rapid changes in areas such as animal traits and management. Despite gains in productivity, often accompanied by reduced emissions intensity and land use extent, the livestock sector: i) is the largest anthropogenic emitter of methane, ii) has significantly reduced global carbon storage and photosynthetic capacities, iii) releases nitrogen and phosphorus to air, water, and/or soil, and iv) also has additional impacts, e.g. antibiotic resistance and biodiversity loss. This chapter explores some of the many facets of livestock’s contributions to climate change and the difficulties involved in quantifying them, with a closer look into one area where scientific consensus has not been reached—the contribution of livestock methane emissions to changing atmospheric methane concentrations over the last few decades. Sections are included on livestock methane (also covering global atmospheric concentrations, and the global methane cycle), and also quantifying enteric fermentation (also covering manure management methane emissions). A case study is also included.