One of the key challenges for integrative analysis is the lack of sufficient metadata. The current version of the "Metadata: Placental Tissues" database contains metadata on human placenta samples. The data were manually standardized using MeSH and EFO ontologies and enriched by information from the articles and personal correspondence with their authors. Raw data on gene expression are available via BioStudies (ArrayExpress) and GEO Omnibus public repositories. Accession numbers for samples and experiments are present in the "Samples" and "Experiments" tables, respectively. The "Attribute Coverage" histogram shows the percentage of samples per available metadata attributes.