Methane is the second-most important anthropogenic greenhouse gas. Ruminant livestock are a major source of emissions. The expansion of pasture is one of the main drivers of deforestation, one of the larger sources of carbon dioxide emissions. An understanding of dietary choice is needed for scenario building and for assessing policy options. This paper focuses on those who eat no meat whatsoever. We estimate that there are one and half billion vegetarians. Only 75 million are vegetarians of choice, a number that will gradually grow with increasing affluence and education. The other 1,450 million are vegetarians of necessity. They will start to eat meat as soon as they can afford it.