In this first part of this paper we review the definition of eosinophilia and their classification according to the degree of elevation of eosinophils/microL. Aetiological factors related with eosinophilia were described in three groups of patients: a). autochthonous non-infected by HIV; b). HIV-infected; and c). arrived from tropical countries (imported eosinophilia). We included an algorithmic approach to the diagnosis, including the diagnostic studies that should be performed in patients with or without organ involvement. Pathological consequences of eosinophilia are indicated in the next part of the paper. Finally, therapeutical options used in patients with eosinophilia are reviewed, with an special emphasis on antihelminthic therapies and the management of the hypereosinophilic syndromes.