As a food, feed and biofuel crop, sorghum is an essential component of global crop production. Worldwide, sorghum is the fifth most important grain crop, and annual world production and the area planted to sorghum have both decreased marginally over the past 30 years from 62.8 to 59.3 million metric tons and from 44.5 to 41.9 million hectares (https://www.cgiar.org/our-strategy/crop-factsheets/sorghum/). Recently, sorghum has become the number two crop for grain-based ethanol in the United States after maize. Sorghum (Sorghum bicolor (L.) Moench) is believed to have been first domesticated in northeastern Africa (Southern Sudan and Ethiopia) (De Wet and Huckabay, 1967; Doggett, 1988) and has spread to tropical and subtropical regions of all continents. This crop has enormous diversity within the National Genetic Resources Program (NGRP), maintaining 43, 077, accessions of S. bicolor subsp. bicolor that can be found in the Germplasm Resource Information System (GRIN). Because sorghum is rich in genetic diversity it is employed for various purposes. In addition to serving as an important dietary staple for more than 500 million people, sorghum is used for animal forage, shelter and recently as a feedstock for biofuel production. One of the key characteristics of sorghum is its ability to yield across a range of environments. Primarily grown in arid and semi-arid regions in the United States, Asia and sub-Saharan Africa, sorghum has been adapted for these harsh environments, representing perhaps the best cereal model for abiotic stress tolerance studies (Borrell et al., 1999; Doggett, 1988; Rosenow and Clark, 1982; Sanchez et al.,

2002; Walulu et al., 1994). Although sorghum exhibits better abiotic stress tolerances than many crops, significant sorghum yield losses are experienced annually in response to unfavourable environments. Globally, domesticated sorghum is often grown on marginal lands and in environments exposing the plant to temperature extremes and water deficits. There are numerous reviews on sorghum abiotic stress tolerance (Mickelbart et al., 2015; Mittler, 2006; Prasad et al., 2015; Sanchez et al., 2002; Wang, 2015), and so much of that information will not be repeated here. This chapter will focus on (1) germplasm diversity in response to low temperatures and water-deficit stress, and (2) methods for identifying diversity among germplasm collections.