Inoculation of animals via inhaled aerosols has long been used to study the infectivity and pathogenesis of both influenza virus and other respiratory pathogens in a context that mimics natural infection. In contrast, traditional in vitro studies of cellular tropism have been limited to the use of liquid inocula. We have recently shown that cultured cells can become successfully infected after exposure to aerosolized influenza virus. In this chapter, we describe the methodology employed, including the operation of aerosolization instrumentation and calculation of infectious dose, both in experimental planning and after exposure occurs.