Case sensitivity refers to a program's ability to distinguish between uppercase, or capital, letters and lowercase, or small, letters. Such programs are described as being case sensitive.

A case sensitive program requires that certain information, often that denotes a task, must be entered in the expected case. If an uppercase entry is required, "RUN" for example, the entry of a lower case "run" could produce a different consequence than the intended one. Some search engines are case sensitive; different and erroneous results can be a consequence of the improper use of case.

Different operating systems utilize case sensitivity in different fashions. DOS does not preserve the case when saving file names in memory, whereas Unix does. In Unix, the retrieval of files is also case sensitive. Case sensitivity is also important in programming.

In Internet protocol, Universal Resource Locators (URLs) are case sensitive. Thus, if the URL consists of upper and lowercase characters, these must be correctly supplied. Otherwise, the URL cannot be located. A "404 Not Found" error message when attempting to reach a URL can arise from the improper use of case.