Giant viruses and virophages are two groups of DNA viruses that infect single-celled eukaryotes (protists). Encoding hundreds of proteins and featuring particles that are visible by light microscopy, giant viruses are the largest known viruses. Their enormous coding potential renders them host-independent for many biochemical pathways, such as transcription, glycosylation, DNA replication and repair, and allows certain giant viruses to replicate entirely in the cytoplasm. Virophages are smaller DNA viruses that parasitize upon the enzymatic complexity of giant viruses. In co-infected host populations, the virophage inhibits replication of the giant virus and increases host survival. We are interested in the underlying mechanisms of virophage-virus-host interactions and in the diversity and evolutionary history of these viruses.