Viruses use different strategies to interact with their host and perform a successful viral infection that results in the formation of new infectious viral particles and their propagation to new hosts. Understanding how viruses interact with their hosts requires the use of high-resolution techniques for the direct visualization of these interactions. Here electron microscopy (EM) methods are described that allow the 3D ultrastructural analysis of virus-infected cells. These methods can be implemented with light microscopy (LM) to certainly allocate virus-infected cells or cells displaying a specific/interesting phenotype caused by the interaction of viral proteins with the cellular machinery. Some sample preparation procedures where LM is integrated, known as correlative light electron microscopy (CLEM), are also explained in this chapter. All of these methods are applicable to any kind of cultured cells, including influenza virus-infected cells.