Since the first characterisation of the eosinophil, it has been associated with various allergic diseases as well as parasitic infections. The ability to manipulate eosinophils levels in mice has led to new perspectives regarding the roles of these cells in multiple diseases from asthma to cancer, as well as in health including metabolism and innate immunity. With the penetrance of potent eosinophil-depleting therapeutic agents it is important to scrutinise these potential roles as predicted from experimental mouse models alongside clinical observations from eosinophil-depleted patients. In this chapter, we comprehensively compare these data. Although no clinically important homeostatic role has been observed, so far, from eosinophil-depleted patients, the roles of eosinophils in health and disease require further study.