Publisher Summary This chapter discusses the etiology/pathogenesis, clinical features, diagnosis, and treatment of Henoch–Schönlein purpura nephritis (HSPN). The classification of pathologic glomerular changes in HSPN is based on endo- and extracapillary inflammation of the glomerulus and bears strong similarity to glomerular lesions observed in systemic vasculitis. Increased immunoglobulin A (IgA) plasma concentrations have been found in various percentages of patients in HSPN. This increase mainly involves polymeric IgA1. Qualitative abnormalities of IgA molecules such as IgA with rheumatoid-factor activity and IgA anti-alpha galactosyl, which might favor IgA-containing complexes (IgA-CC) formation, have also been found in HSPN. Increased formations of circulating IgA-CC in HSPN can be the consequence of transmucosal penetration of exogenous antigens. The intestinal permeability to 51Cr EDTA is increased in HSPN. Remissions of HSPN in adults have been reported during pregnancy or sex-hormone therapy. The latter observation as well as the mean sex ratio (M/F: 1:5) is in favor of a pathogenic role of male hormones.