pˌɑːpjʊlˈeɪʃən mˈæɾɚz, fˈɔːɹmɚli nˈoʊn æz ðɪ ˈɑːptɪməm pˌɑːpjʊlˈeɪʃən tɹˈʌst, ɪz ɐ jˌuːkˈeɪbˈeɪst tʃˈæɹᵻɾi ðæt ɐdɹˈɛsᵻz pˌɑːpjʊlˈeɪʃən sˈaɪz ænd ɪts ɪfˈɛkts ˌɔn ɛnvˌaɪɹənmˈɛntəl səstˌeɪnəbˈɪlᵻɾi.
Population Matters, formerly known as the Optimum Population Trust, is a UK-based charity that addresses population size and its effects on environmental sustainability.