A Globally Unique Identifier or GUID (pronounced /ˈguːɪd/ or /ˈgwɪd/) is a special type of identifier used in software applications in order to provide a reference number which is unique in any context (hence, "Globally"), for example, in defining the internal reference for a type of access point in a software application, or for creating unique keys in a database. While each generated GUID is not guaranteed to be unique, the total number of unique keys (2¹²⁸ or 3.4×10³⁸) is so large that the probability of the same number being generated twice is very small. For example, consider the observable universe, which contains about 5×10²² stars; every star could then have 6.8×10¹⁵ universally unique GUIDs.

The term GUID usually refers to Microsoft's implementation of the Universally Unique Identifier (UUID) standard. Many systems use the term GUID, however, including Oracle Database, MySQL, dBase, OpenView Operations, ISIS Papyrus, and Novell eDirectory. The GUID is also the basis of the GUID Partition Table, Intel's replacement for Master Boot Records under EFI.

The GUID is a 16-byte (128-bit) number. The most commonly used structure of the data type is:

Bits	Bytes	Description
32	4	Data1
16	2	Data2
16	2	Data3
64	8	Data4

Except for Data4, the most significant byte in every field is stored last; Data4 stores all bytes consecutively.

One to three of the most significant bits of the second byte in Data 4 define the type variant of the GUID:

Pattern	Description
000	Network Computing System backward compatibility
100	Standard
110	Microsoft Component Object Model backward compatibility; this includes the GUID's for important interfaces like IUnknown and IDispatch.
111	Reserved for future use.

The most significant four bits of Data3 define the version number, and the algorithm used.

Text encoding

Guids are most commonly written in text as a sequence of hexadecimal digits such as:

3F2504E0-4F89-11D3-9A0C-0305E82C3301

This text notation contains the following fields, separated by hyphens:

Hex digits	Description
8	Data1
4	Data2
4	Data3
4	Initial two bytes from Data4
12	Remaining six bytes from Data4

For the first three fields, the most significant digit is on the left. The last two fields are treated as eight separate bytes, each having their most significant digit on the left, and they follow each other from left to right. Note that the digit order of the fourth field may be unexpected, since it's treated differently than in the structure.

Often braces are added to enclose the above format, as such:

{3F2504E0-4F89-11D3-9A0C-0305E82C3301}

When printing fewer characters is desired, GUIDs are sometimes encoded into a base64 or Ascii85 string. Base64-encoded GUID consists of 22 to 24 characters (depending on padding), for instance:

7QDBkvCA1+B9K/U0vrQx1A

7QDBkvCA1+B9K/U0vrQx1A==

and Ascii85 encoding gives only 20 characters, e. g.:

5:$Hj:Pf\4RLB9%kU\Lj