This chapter describes a basic workflow for analyzing the protein composition of influenza virions. In order to obtain suitable material, the chapter describes how to concentrate influenza virions from the growth media of infected cells and to purify them by ultracentrifugation through a density gradient. This approach is also suitable for purifying influenza virions from the allantoic fluid of embryonated chicken eggs. As a small quantity of microvesicles are co-purified with virions, optional steps are included to increase the stringency of purification by enriching material with viral receptor binding and cleaving activity. Material purified in this way can be used for a variety of downstream applications, including proteomics. As a detailed example of this, the chapter also describes a standard workflow for analyzing the protein composition of concentrated virions by liquid chromatography and tandem mass spectrometry.