|Key people||Simon Ross (CEO)|
|Focus(es)||Impact of population growth|
|Motto||"For a sustainable future"|
Population Matters, formerly known as the Optimum Population Trust, is a registered United Kingdom charity, think tank, and campaign group concerned with the impact of population growth on long term sustainability, quality of life and the natural environment, specifically natural resources, climate change, and biodiversity.
Population Matters researches climate change, energy requirements, biodiversity, and other environmental factors in relation to population numbers. It campaigns for population stabilisation and gradual decrease to sustainable levels for both the world and the United Kingdom. In 2009, the organisation issued a study asserting that contraception was the cheapest way of combating climate change. 
The trust states that its intermediate aims are: improved provision of family planning and sex education, better education and rights for women, and that couples voluntarily "have two or fewer". For the UK specifically, it advocates greater effort to reduce the high rates of teenage pregnancy and unintended pregnancy and that immigration be brought into balance with emigration.
The Optimum Population Trust was founded in 1991 by the late David Willey. It was granted charitable status on 9 May 2006. The working name was changed to Population Matters in February 2011.
The current chair is Roger Martin. Past chairs, in chronological order, were:
- David Willey
- Edmund Davey
- Rosamund McDougall and John Guillebaud (co-chairs)
- John Guillebaud
- Val Stevens
- Sue Birley
- Val Stevens.
- Sir David Attenborough OM CH CVO CBE, Naturalist, broadcaster and trustee of the British Museum and Royal Botanical Gardens, Kew; and a former controller of BBC Two
- Professor Sir Partha Dasgupta, Frank Ramsey Professor of Economics, University of Cambridge
- Professor Paul Ehrlich, Bing Professor of Population Studies, Stanford University
- Baroness Shreela Flather, politician and past mayor of the Royal Borough of Windsor and Maidenhead
- Dr Jane Goodall DBE, Founder, Jane Goodall Institute, and UN Messenger of Peace
- Susan Hampshire OBE, Actress and population campaigner
- Professor John Guillebaud, Emeritus Professor of Family Planning and Reproductive Health, University College, London.
- Dr James Lovelock CBE, Scientist and environmentalist known for proposing the Gaia theory that Earth functions as an organism, and author of 'The Revenge of Gaia'
- Professor Aubrey Manning OBE, Emeritus Professor of Natural History, University of Edinburgh
- Professor Norman Myers CMG, Visiting Fellow, Green College, Oxford University, and at Harvard University, Cornell University, Stanford University, University of California, Berkeley, Michigan University and University of Texas
- Chris Packham, Naturalist, nature photographer, television presenter and author.
- Sara Parkin OBE, Founder Director and Trustee of Forum for the Future, Chair of the Richard Sandbrook Trust, Board member of the European Training Foundation
- Jonathon Porritt CBE, Founder Director of Forum for the Future and former Chair of the UK Sustainable Development Commission
- Lionel Shriver Journalist and author
- Sir Crispin Tickell GCMG KCVO, Chancellor of Kent University, Director of the Policy Foresight Programme at the James Martin Institute, and former UK Permanent Representative on the United Nations Security Council
- Official website
- Letter to New Scientist from David Willey (Subscription only)
- Guardian article: Citizens arrest
- Independent article: Overpopulation is main threat to planet
- Telegraph article: UK unable to sustain population, says study
- New Statesman article: Planet Overload
- Times article: David Attenborough to be patron of Optimum Population Trust
- Times article: Having large families is an 'eco-crime'