Influenza virosomes are an efficient antigen carrier and adjuvant system that are approved for the use in human vaccines. Structurally, virosomes are spherical vesicles of approximately 150 nm in diameter, composed of a lipid membrane with integrated envelope proteins derived from influenza virus, predominantly hemagglutinin. The particle structure, together with the functions of hemagglutinin--receptor binding, pH-dependent fusion activity and immunostimulation--is responsible for the adjuvant effect of virosomes. In contrast to most other virus-like particles, virosomes are semisynthetic particles reconstituted in vitro from lipids and from virus-derived proteins. The production process has proven to be robust at industrial scale and fully compatible with Good Manufacturing Practice guidelines. At the same time, the formulation procedure is sufficiently flexible to allow modifications of the composition and structure for the intended use, including the positioning of the antigens of interest.