Although several virus families are internalized into their host cells by direct fusion of the viral envelope with the plasma membrane, most viruses, for example, influenza virus, make use of endocytic pathways for productive entry and infection. After endocytosis, the influenza virus escapes from the endocytic compartment to the cytosol. The distribution of the incoming influenza virus could be traced by detection of the viral RNA in the distinct cellular compartments, including endosome, cytosol, and nucleus. To accomplish this work, we developed a subcellular fractionation method based on density gradient ultracentrifugation and detected the viral RNA using quantitative reverse transcription-polymerase chain reaction analysis. This chapter is devoted to the practical methods and precautions for studying endocytic traffic of virus as well as host cellular factors affecting viral endocytosis.