Updated project metadata. ISG15 is an interferon-stimulated, ubiquitin-like protein, with anti-viral activity against several clinically relevant viruses. However, its precise mechanism of action during viral infection remains elusive. Upon infection, ISG15 conjugates to protein substrates in covalent manner. We previously found that ISG15 restricts Coxsackie virus (CV) infection both in vitro and in vivo. Here, we endeavored to map the in vivo ISGylome following CV infection to mechanistically elucidate the function of ISG15 in host defense. To do so we combined a genetic approach employing a murine model of deregulated ISGylation with quantitative proteomics of immune-enriched endogenous ISG15 modification sites. In addition, we also quantified protein level changes in the host proteome following infection.