Eosinophilia represents a complex clinical problem, the management of which is based on case history and clinical examination. Allergy, parasitic infection or inflammation may then be identified or malignancy suspected. In some cases, representing a haematologic disorder, clonality is demonstrated only by specific tests. The diagnostic and therapeutic approach, including a new classification for eosinophilia, is reviewed, and the importance of molecular biological technique is highlighted. Unexplained eosinophilia should be managed in collaboration with a haematology department.