The Pathogen-Host Interactions database (PHI-base) is a biological database that contains curated information on genes experimentally proven to affect the outcome of pathogen-host interactions. The database is maintained by researchers at Rothamsted Research, together with external collaborators since 2005. Since April 2017 PHI-base is part of ELIXIR, the European life-science infrastructure for biological information via its ELIXIR-UK node. Background The Pathogen-Host Interactions database was developed to utilise effectively the growing number of verified genes that mediate an organism's ability to cause disease and / or to trigger host responses. The web-accessible database catalogues experimentally verified pathogenicity, virulence and effector genes from bacterial, fungal and oomycete pathogens which infect animal, plant and fungal hosts. PHI-base is the first on-line resource devoted to the identification and presentation of information on fungal and oomycete pathogenicity genes and their host interactions. As such, PHI-base is a valuable resource for the discovery of candidate targets in medically and agronomically important fungal and oomycete pathogens for intervention with synthetic chemistries and natural products (fungicides). Each entry in PHI-base is curated by domain experts and supported by strong experimental evidence (gene disruption experiments) as well as literature references in which the experiments are described. Each gene in PHI-base is presented with its nucleotide and deduced amino acid sequence as well as a detailed structured description of the predicted protein's function during the host infection process. To facilitate data interoperability, genes are annotated using controlled vocabularies (Gene Ontology terms, EC Numbers, etc.), and links to other external data sources such as UniProt, EMBL and the NCBI taxonomy services. Current developments Version 4.11 (May 5th, 2021) of PHI-base provides information on 8070 genes from 276 pathogens and 224 hosts and their impact on 17060 interactions as well on efficacy information on ~20 drugs and the target sequences in the pathogen. PHI-base currently focuses on plant pathogenic and human pathogenic organisms including fungi, oomycetes and bacteria. The entire contents of the database can be downloaded in a tab delimited format. Since 2015 the web-site includes an online literature curation tool called PHI-Canto for community literature curation of various pathogenic species. Since the launch of version 4, the PHI-base is also searchable using the PHIB-BLAST search tool, which uses the BLAST algorithm to compare a user's sequence against the sequences available from PHI-base. In 2016 the plant portion of PHI-base was used to establish a Semantic PHI-base search tool". PHI-base is