In this chapter, the authors first explore the variety of ways in which eating can depart from those behaviours which support good health. They then summarise each of the dominant models that have been proposed to explain and predict unhealthy eating. The authors present a social identity approach to eating behaviour along with evidence that speaks to its usefulness. People sometimes mistakenly think that over-eating and under-eating behaviours, and the disorders they pre-empt, are so different from one another that they cannot be understood using the same theories or approaches. Research has shown that rates of comorbid obesity and eating disorders are increasing faster than either condition alone. The incidence of unhealthy eating also varies across the spectrum of eating disorders. The key benefit of biological models is that they can help identify those individuals in a given population who are most at risk of engaging in unhealthy eating behaviour.