Human cytomegalovirus (HCMV) is a ubiquitous and clinically significant herpesvirus that causes substantial morbidity in immunocompromised individuals and during congenital infection. Available therapies suffer from the potential for drug resistance and significant side effects, and a vaccine is not yet available. A comprehensive understanding of the functions of viral proteins is therefore critical for the rational design of new antiviral drugs. We previously found that HCMV manipulates the expression of thousands of host proteins, including downregulation/degradation of known and novel antiviral restriction factors and cell death pathway components. Metabolic and biosynthetic pathways were upregulated to favour productive viral infection. However, a systematic understanding of which viral factor modulates each host protein is currently lacking. We have developed a systematic CRISPR-Cas9 knockout screen to discover the functions of every HCMV protein. We established 180 cell lines, each co-expressing Cas9 and guide RNAs targeting one-or-other of 168 canonical HCMV proteins and 12 novel ORFs, then infected these with HCMV. 18-plex tandem mass tag-based proteomics and RNA sequencing enabled a systematic comparative analysis of gene function at the level of host protein and transcript, revealing both global and individual insights into viral protein function. This identified novel functions for multiple previously uncharacterised HCMV ORFs. In addition to presenting data from this knockout screen and validation studies, I will introduce insights into the overlap between this data and our previous analysis of HCMV ‘interactome’, leading to a unified model of HCMV protein function.