IgG4-related disease is a protean condition that mimics many malignant, infectious, and inflammatory disorders. This multi-organ immune-mediated condition links many disorders previously regarded as isolated, single-organ diseases without any known underlying systemic condition. It was recognised as a unified entity only 10 years ago. Histopathology is the key to diagnosis. The three central pathology features of IgG4-related disease are lymphoplasmacytic infiltration, storiform fibrosis, and obliterative phlebitis. The extent of fibrosis is an important determinant of responsiveness to immunosuppressive therapies. IgG4-related disease generally responds to glucocorticoids in its inflammatory stage, but recurrent or refractory cases are common. Important mechanistic insights have been derived from studies of patients treated by B-cell depletion. Greater awareness of this disease is needed to ensure earlier diagnoses, which can prevent severe organ damage, disabling tissue fibrosis, and even death. Identification of specific antigens and T-cell clones that drive the disease will be the first steps to elucidate the pathogenesis of IgG4-related disease.