Medical Subject Headings
Medical Subject Headings (MeSH) is a huge controlled vocabulary (or metadata system) for the purpose of indexing journal articles and books in the life sciences. Created and updated by the United States National Library of Medicine (NLM), it is used by the MEDLINE/PubMed article database and by NLM's catalog of book holdings. MeSH can be browsed and downloaded free of charge on the Internet. The yearly printed version was discontinued in 2007.
In MEDLINE/PubMed, every journal article is indexed with some 10-15 headings or subheadings, with one or two of them designated as major and marked with an asterisk. When performing a MEDLINE search via PubMed, entry terms are automatically translated into the corresponding descriptors (see Details tab in PubMed). Furthermore, by default all the descriptors below the given one in the hierarchy are included in the search.

# Structure of MeSH
The 2005 version of MeSH contains a total of 22,568 subject headings, also known as descriptors. Most of these are accompanied by a short definition, links to related descriptors, and a list of synonyms or very similar terms (known as entry terms). Because of these synonym lists, MeSH can also be viewed as a thesaurus.

## Descriptors
The descriptors are arranged in a hierarchy. A given descriptor may appear at several places in the hierarchical tree. The tree locations carry systematic labels known as tree numbers, and consequently one descriptor can carry several tree numbers. For example, following the graphic at the top right, C stands for Diseases, C06 for Digestive System Diseases and C06.301 for Digestive System Neoplams; C04 for Neoplasms, C04.588 for Neoplasms By Site, and C04.588.274 is a second tree number for Digestive System Neoplasms. As seen from the graphic, Stomach Neoplasms appears in four locations in the hierarchy and therefore carries four different tree numbers. The tree numbers of a given descriptor are subject to change as MeSH is updated. Every descriptor also carries a unique alphanumerical ID that will not change.

## Description
Most entries come with a short description. See the MeSH description for diabetes type 2 as an example. The explanatory text is written by the MeSH team based on their standard sources  if not otherwise stated. References are mostly encyclopaedias and standard textbooks of the subject areas. Sources are subdivided into categories . However, references for specific statements in the descriptions are not given, instead readers are referred to the bibliography.

## Qualifiers
In addition to the descriptor hierarchy, MeSH contains a small number of standard qualifiers (also known as subheadings), which can be added to descriptors to narrow down the topic. For example, "Measles" is a descriptor and "epidemiology" is a qualifier; "Measles/epidemiology" describes the subheading of epidemiological articles about Measles. The "epidemiology" qualifier can be added to all other disease descriptors. Not all descriptor/qualifier combinations are allowed since some of them may be meaningless. In all there are 83 different qualifiers.

## Supplements
In addition to the descriptors, MeSH also contains some 139,000 Supplementary Concept Records. These do not belong to the controlled vocabulary as such and are not used for indexing MEDLINE articles; instead they enlarge the thesaurus and contain links to the closest fitting descriptor to be used in a MEDLINE search. Many of these records describe chemical substances.
MeSH has been translated into numerous other languages and allows retrieval of documents from different languages.

# Top level categories
The top level categories in the MeSH descriptor hierarchy are:
- Anatomy 
- Template:MeshNumber --- body regions
- Template:MeshNumber --- musculoskeletal system
- Template:MeshNumber --- digestive system
- Template:MeshNumber --- respiratory system
- Template:MeshNumber --- urogenital system
- Template:MeshNumber --- endocrine system
- Template:MeshNumber --- cardiovascular system
- Template:MeshNumber --- nervous system
- Template:MeshNumber --- sense organs
- Template:MeshNumber --- biological tissues
- Template:MeshNumber --- cells
- Template:MeshNumber --- fluids and secretions
- Template:MeshNumber --- animal structures
- Template:MeshNumber --- stomatognathic system
- Template:MeshNumber --- hemic and immune systems
- Template:MeshNumber --- embryonic structures
- Template:MeshNumber --- integumentary system
- Organisms 
- Template:MeshNumber --- animals
- Template:MeshNumber --- algae
- Template:MeshNumber --- bacteria
- Template:MeshNumber --- viruses
- Template:MeshNumber --- fungi
- Template:MeshNumber --- plants
- Template:MeshNumber --- archaea
- Template:MeshNumber --- mesomycetozoea
- Diseases 
- Template:MeshNumber --- bacterial infections and mycoses
- Template:MeshNumber --- virus diseases
- Template:MeshNumber --- parasitic diseases
- Template:MeshNumber --- neoplasms
- Template:MeshNumber --- musculoskeletal diseases
- Template:MeshNumber --- digestive system diseases
- Template:MeshNumber --- stomatognathic diseases
- Template:MeshNumber --- respiratory tract diseases
- Template:MeshNumber --- otorhinolaryngologic diseases
- Template:MeshNumber --- nervous system diseases
- Template:MeshNumber --- eye diseases
- Template:MeshNumber --- urologic and male genital diseases
- Template:MeshNumber --- female genital diseases and pregnancy complications
- Template:MeshNumber --- cardiovascular diseases
- Template:MeshNumber --- hemic and lymphatic diseases
- Template:MeshNumber --- congenital, hereditary, and neonatal diseases and abnormalities
- Template:MeshNumber --- skin and connective tissue diseases
- Template:MeshNumber --- nutritional and metabolic diseases
- Template:MeshNumber --- endocrine system diseases
- Template:MeshNumber --- immune system diseases
- Template:MeshNumber --- disorders of environmental origin
- Template:MeshNumber --- animal diseases
- Template:MeshNumber --- pathological conditions, signs and symptoms
- Chemicals and Drugs 
- Template:MeshNumber --- inorganic chemicals
- Template:MeshNumber --- organic chemicals
- Template:MeshNumber --- heterocyclic compounds
- Template:MeshNumber --- polycyclic compounds
- Template:MeshNumber --- macromolecular substances
- Template:MeshNumber --- hormones, hormone substitutes, and hormone antagonists
- Template:MeshNumber --- enzymes and coenzymes
- Template:MeshNumber --- carbohydrates
- Template:MeshNumber --- lipids
- Template:MeshNumber --- amino acids, peptides, and proteins
- Template:MeshNumber --- nucleic acids, nucleotides, and nucleosides
- Template:MeshNumber --- complex mixtures
- Template:MeshNumber --- biological factors
- Template:MeshNumber --- biomedical and dental materials
- Template:MeshNumber --- pharmaceutical preparations
- Template:MeshNumber --- chemical actions and uses
- Analytical, Diagnostic and Therapeutic Techniques and Equipment 
- Template:MeshNumber --- diagnosis
- Template:MeshNumber --- therapeutics
- Template:MeshNumber --- anesthesia and analgesia
- Template:MeshNumber --- surgical procedures, operative
- Template:MeshNumber --- investigative techniques
- Template:MeshNumber --- dentistry
- Template:MeshNumber --- equipment and supplies
- Psychiatry and Psychology 
- Template:MeshNumber --- behavior and behavior mechanisms
- Template:MeshNumber --- psychological phenomena and processes
- Template:MeshNumber --- mental disorders
- Template:MeshNumber --- behavioral disciplines and activities
- Biological Sciences 
- Template:MeshNumber --- biological sciences
- Template:MeshNumber --- health occupations
- Template:MeshNumber --- environment and public health
- Template:MeshNumber --- biological phenomena, cell phenomena, and immunity
- Template:MeshNumber --- genetic processes
- Template:MeshNumber --- biochemical phenomena, metabolism, and nutrition
- Template:MeshNumber --- physiological processes
- Template:MeshNumber --- reproductive and urinary physiology
- Template:MeshNumber --- circulatory and respiratory physiology
- Template:MeshNumber --- digestive, oral, and skin physiology
- Template:MeshNumber --- musculoskeletal, neural, and ocular physiology
- Template:MeshNumber --- chemical and pharmacologic phenomena
- Template:MeshNumber --- genetic phenomena
- Template:MeshNumber --- genetic structures
- Physical Sciences 
- Template:MeshNumber --- natural sciences
- Anthropology, Education, Sociology and Social Phenomena 
- Template:MeshNumber --- social sciences
- Template:MeshNumber --- education
- Template:MeshNumber --- human activities
- Technology and Food and Beverages 
- Template:MeshNumber --- technology, industry, and agriculture
- Template:MeshNumber --- food and beverages
- Humanities 
- Template:MeshNumber --- humanities
- Information Science 
- Template:MeshNumber --- information science
- Persons 
- Template:MeshNumber --- persons
- Health Care 
- Template:MeshNumber --- population characteristics
- Template:MeshNumber --- health care facilities, manpower, and services
- Template:MeshNumber --- health care economics and organizations
- Template:MeshNumber --- health services administration
- Template:MeshNumber --- health care quality, access, and evaluation
- Publication Characteristics 
- Template:MeshNumber --- publication components (publication type)
- Template:MeshNumber --- publication formats (publication type)
- Template:MeshNumber --- study characteristics (publication type)
- Template:MeshNumber --- support of research
- Geographic Locations 
- Template:MeshNumber --- geographic locations