The MeSH (Medical Subject Headings) thesaurus is a controlled vocabulary produced by the NLM and used for indexing, cataloging, and searching for biomedical and health-related information and documents. MeSH includes the subject headings appearing in MEDLINE/PubMed, the NLM Catalog, and other NLM databases. MeSH is also used by ClinicalTrials.gov to classify which diseases are studied by trials registered in the database.

MeSH organizes its descriptors (main headings) in a hierarchical structure so that broad searches will find articles indexed more narrowly. This structure also provides an effective way for searchers to browse MeSH to find appropriate descriptors. Many synonyms, near-synonyms, and closely related concepts are included as entry terms to help users find the most relevant MeSH descriptor for the concept they are seeking. In NLM's online databases, many terms entered by searchers are automatically mapped to MeSH descriptors to facilitate retrieval of relevant information.

MeSH descriptors are organized in 16 categories: category A for anatomic terms, category B for organisms, C for diseases, D for drugs and chemicals, etc. Each category is further divided into subcategories. Within each subcategory, descriptors are arrayed hierarchically from most general to most specific in up to thirteen hierarchical levels. Because of the branching structure of the hierarchies, these lists are sometimes referred to as "trees." Each MeSH descriptor appears in at least one place in the trees and may appear in as many additional places as may be appropriate. Those who use MeSH should find the most specific MeSH descriptor that is available to represent each concept of interest.

For example, articles concerning Streptococcus pneumoniae will be found under the descriptor "Streptococcus Pneumoniae" rather than the broader term Streptococcus, while an article referring to a new streptococcal bacterium which is not yet in the vocabulary will be listed directly under Streptococcus. Accordingly, the user may consult the trees to find additional terms which are more specific than a given heading, and broader headings as well.

For example, under Abnormalities, there are specific abnormalities:

	Congenital Abnormalities C16.131
	Abnormalities, Drug Induced C16.131.042s
	Abnormalities, Multiple C16.131.077
		22q11 Deletion Syndrome C16.131.077.019
			DiGeorge Syndrome C16.131.077.019.500