A file signature is data used to identify or verify the content of a file. Such signatures are also known as magic numbers or magic bytes and are usually inserted at the beginning of the file.

Many file formats are not intended to be read as text. If such a file is accidentally viewed as a text file, its contents will be unintelligible. However, some file signatures can be recognizable when interpreted as text. In the table below, the column "ISO 8859-1" shows how the file signature appears when interpreted as text in the common ISO 8859-1 encoding, with unprintable characters represented as the control code abbreviation or symbol, or codepage 1252 character where available, or a box otherwise. In some cases the space character is shown as ␠.