People have been worrying about how many people the Earth can support for thousands of years. In the last 350 years, scientists have published more than 60 estimates of the number of people the Earth can support. These estimates range from fewer than 1 billion to more than 1,000 billion. The nature and limitations of the mathematical methods used to derive these estimates will be described. Earth’s capacity to support people is determined both by natural constraints and by human choices concerning economics, environment, culture (including values and politics), and demography. Human carrying capacity is therefore dynamic and uncertain. Human choice is not captured by ecological notions of carrying capacity that are appropriate for non-human populations. How many people the Earth can support has been and will be strongly influenced by how humans choose to interact with one another and with the Earth.