Code 128 was introduced in 1981 as a very high-density alphanumeric symbology. Since 1990, it has seen increasing adoption in a variety of applications. It has the ability to adapt itself to all-numeric data or to alphanumeric data. This is done with "start" and "shift" codes specifying that the data following is numeric, alphanumeric, or special characters. These compression techniques minimize space while allowing for numeric and alphanumeric data. Because of the compact symbol, the numeric version is used whenever possible, but even alphanumeric Code 128 is more compact than Code 39. It is a variable length, continuous symbology with multiple element width. It is often selected over Code 39 in new applications because of its density and because it offers a much larger selection of characters.

The Code 128 character set includes the digits 0-9, the letters A-Z (upper and lower cases), and all standard ASCII symbols and control codes.

The codes are divided into three subsets A, B, and C. There are three separate start codes to indicate which subset will be used; in addition, each subset includes control characters to switch to another subset in the middle of a barcode.

• Code 128A includes the standard ASCII symbols, digits, upper case letters, and control codes.
• Code 128B includes standard ASCII symbols, digit, upper and lower case letters.
• Code 128C compresses two numeric digits into each character, providing excellent density.

Each character consists of 3 bars and 3 spaces, each of which may be 1, 2, or 3 elements wide (1 element = 1/11th of the character width). The bars always use an even number of elements and the spaces use an old number. This provides the basis for a character-by-character consistency check during scanning. Each Code 128 barcode include a Modulo 103 checksum.

UCC/EAN 128 (called only EAN 128 to make it short)

The EAN 128 ( Europäische ArtikelNummerierung) is a special form of the Code 128. EAN 128 is based on Code128 but has a double start character.