Immunoglobulin G4-related disease (IgG4-RD) is a heterogeneous immune-mediated condition that can affect almost any organ and is now being recognised with increasing frequency. Laboratory abnormalities including peripheral eosinophilia, hypergammaglobulinaemia, elevated serum IgE level, and hypocomplementaemia often provide initial clues to the diagnosis of IgG4-RD. The distinctive histopathological hallmarks of IgG4-RD are a dense lymphoplasmacytic infiltration with a high percentage of IgG4+ plasma cells, storiform fibrosis, obliterative phlebitis, and mild to moderate tissue eosinophilia. Around 20-40% of patients with IgG4-RD presented with peripheral eosinophilia and 51-86% are manifested as tissue eosinophilia. These data indicate an extensive involvement of eosinophil in IgG4-RD. Here, we review the biology of eosinophil, the discovery of eosinophilia in IgG4-RD, and its association with disease activity and relapse. We also discuss the possible functions and therapeutic potential of eosinophil in IgG4-RD.