Biological sequence diversity is nowhere as apparent as in the vast sequence space of viral genomes. The VIRUS-X project will specifically explore the outer realms of this diversity by targeting the virosphere of selected microbial ecosystems and investigate the encoded functional variety of viral gene products. The project is driven by the expected large innovation value and unique properties of viral proteins, previously demonstrated by the many virally derived DNA and RNA processing enzymes used in biotechnology. Concomitantly, the project will advance our understanding of important aspects of ecology in terms of viral diversity, ecosystem dynamics and virus-host interplay. Last but not least, due to the inherent challenges in gene annotation, functional assignments and other virus-specific technical obstacles of viral metagenomics, the Virus-X project specifically addresses these challenges using innovative measures in all parts of the discovery and analysis pipeline, from sampling difficult extreme biotopes, through sequencing and innovative bioinformatics to efficient production of enzymes for molecular biotechnology. Virus-X will advance the metagenomic toolbox significantly and our capabilities for future exploitation of viral biological diversity, the largest unexplored genetic reservoir on Earth.