In health, eosinophils have varied physiological roles such as tissue homeostasis, adaptive immunity, pathogen recognition and clearance, and tissue repair. Eosinophilic lung diseases are a heterogeneous collection of conditions that have in common tissue (lung) eosinophilia. Peripheral eosinophilia may be absent. This chapter focuses on a strategic approach to the diagnosis of eosinophilic lung disorders. A detailed discussion includes the causes and the pathobiology of the various pulmonary infiltrations with eosinophilia (PIE) syndromes and focuses on seeking out the diagnosis by systematic examination of these. Also covered in this chapter are the implications of the differing grades of peripheral eosinophilia in relation to lung disease, and a simplified approach to the patient presenting with marked eosinophilia.