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Joining of strings in a programming language
the string operation of computer programming
concatenation of general lists

In formal language theory and computer programming, string concatenation is the operation of joining character strings end to end. For example, the concatenation of "snow" and "ball" is "snowball". In certain formalisations of concatenation theory, also called string theory, string concatenation is a primitive notion.

In many programming languages, string concatenation is a binary infix operator, and in some it is written without an operator. This is implemented in different ways:
Overloading the plus sign + Example from C#: "Hello, " + "World" has the value "Hello, World". Dedicated operator, such as in PHP, & in Visual Basic and || in SQL. This has the advantage over reusing + that it allows implicit type conversion to string. string literal concatenation, which means that adjacent strings are concatenated without any operator. Example from C: "Hello, " "World" has the value "Hello, World".

In programming, string concatenation generally occurs at run time, as string values are typically not known until run time. However, in the case of string literals, the values are known at compile time, and thus string concatenation can be done at compile time, either via string literal concatenation or via constant folding.

The strings over an alphabet, with the concatenation operation, form an associative algebraic structure with identity element the null string—a free monoid. Sets of strings with concatenation and alternation form a semiring, with concatenation (*) distributing over alternation (+); 0 is the empty set and 1 the set consisting of just the null string.

One of the principles of relational database design is that the fields of data tables should reflect a single characteristic of the table's subject, which means that they should not contain concatenated strings. When concatenation is desired in a report, it should be provided at the time of running the report. For example, to display the physical address of a certain customer, the data might include building number, street name, building sub unit number, city name, state/province name, postal code, and country name, e. g., "123 Fake St Apt 4, Boulder, CO 80302, USA", which combines seven fields. However, the customers data table should not use one field to store that concatenated string; rather, the concatenation of the seven fields should happen upon running the report. The reason for such principles is that without them, the entry and updating of large volumes of data becomes error prone and labor intensive. Separately entering the city, state, ZIP code, and nation allows data entry validation (such as detecting an invalid state abbreviation). Then those separate items can be used for sorting or indexing the records, such as all with "Boulder" as the city name.

In recreational mathematics, many problems concern the properties of numbers under concatenation of their numerals in some base. Examples include home primes (primes obtained by repeatedly factoring the increasing concatenation of prime factors of a given number), Smarandache–Wellin numbers (the concatenations of the first prime numbers), and the Champernowne and Copeland–Erdős constants (the real numbers formed by the decimal representations of the positive integers and the prime numbers, respectively).