Njaa ni hali ya kukosa chakula kinachohitajiwa na mwili ili kujimudu. Sababu zake zinaweza kuwa mbalimbali, kama vile vita, ukame, maradhi ya mimea, wadudu waharibifu, utovu wa ustadi katika uzalishaji na utunzaji wa mazao, ugumu wa mawasiliano n.k. Kila bara liliwahi kupatwa na hali hiyo katika historia, na bado hiyo ni tishio kwa nchi mbalimbali, pia kutokana na sababu mpya, kama vile utandawazi, matumizi ya mazao kwa ajili ya kuendesha mashine kiekolojia. Asimov, Isaac, Asimov's New Guide to Science, pp. 152–153, Basic Books, Inc. : 1984. Bhatia, B.M. (1985) Famines in India: A study in Some Aspects of the Economic History of India with Special Reference to Food Problem, Delhi: Konark Publishers Pvt. Ltd. (1984) Agrarian power and agricultural productivity in South Asia 1. University of California Press. ISBN 978-0-520-05369-4. Retrieved on 1 Oktoba 2010.  Davis, Mike, Late Victorian Holocausts: El Niño Famines and the Making of the Third World, London, Verso, 2002 (Excerpt online.) Dutt, Romesh C. Open Letters to Lord Curzon on Famines and Land Assessments in India, first published 1900, 2005 edition by Adamant Media Corporation, Elibron Classics Series, ISBN 1-4021-5115-2.