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The Hessian fly (Mayetiola destructor) is a gall-forming insect (Harris et al. 2006) with several attributes that make it suitable for genetic analysis: a short life cycle (30 days), a small genome (158 Mb) (Johnston et al. 2004), and polytene chromosomes. The reproductive biology and behavior of the insect also make it an attractive model (Harris and Rose 1989,1990; Bergh et al. 1990; Kanno and Harris 2000; Morris et al. 2000; Harris et al. 2001). Compared to most plant-feeding insects, Hessian flies can be reared in a small space. Unlike gall-forming aphid species, the Hessian fly is always sexually reproducing. Females mate only once and deposit 100–400 eggs on the adaxial surfaces of wheat leaves in a short time (approximately 3 hours). Eggs hatch in only 3–4 days at 20°C. Newly hatched larvae move to the base of the nearest node where their feeding causes abnormal stem and leaf growth, stunting, and the eventual death of seedlings (Anderson and Harris 2006). Larvae normally feed for only 10–12 days, and up to 50 larvae can survive on a single wheat seedling. Non-feeding (third instar) larvae can be easily maintained in diapause at 4 °C for more than a year. This makes it possible to conveniently maintain collections of various Hessian fly populations and genotypes without continual breeding. It also makes it possible to screen thousands of wheat plants for resistance to specific genotypes of the pest. Screening has discovered over 30 Hessian fly resistance genes (H genes named H1, H2, H3, etc.) (Martin-Sanchez et al. 2003; Sardesai et al. 2005). It has also permitted the discovery of Hessian fly phenotypes (commonly referred to as biotypes) that differ with respect to their ability to survive on wheat plants carrying different H genes. The desire to understand the mechanisms that underlie these phenotypes remains the major impetus for studying the genetics of this pest. This chapter provides a brief history of these investigations, gives an overview of the current state of Hessian fly genomics, and draws attention to areas for future Hessian fly research.