An essential part of the immune response to viral infections is the recognition and elimination of infected cells by cytotoxic T lymphocytes. For this purpose a display mechanism has evolved which is present in almost all nucleated cells: the major histocompatibility complex class I antigen processing pathway. Both self and foreign antigens are degraded in the cytosol to peptides which are translocated into the endoplasmic reticulum where they are loaded onto MHC class I molecules. Pathogens living inside the cell are evolving under the constant selection pressure of such immune surveillance. As a result such infectious organisms have developed a variety of strategies to prevent their antigens from being presented. Since our understanding of the cell biology of antigen presentation has greatly advanced in recent years, it has now become possible to unravel several of the molecular mechanisms by which viruses interfere with MHC class I antigen presentation. Examples for the interference of viral molecules with components of the MHC class I pathway are presented in this review.