A card catalog is a physical listing of all of the contents of a library, organized with a single card for each item in the library. The card catalog was a familiar navigational hazard and blessing in all libraries well through the late 20th century, when physical card catalogs began to be displaced by computerized versions. Some libraries retain their card catalogs, often as sentimental mementos, and a few actively maintain their card catalogs: this is most common in small, remote libraries.

The need to catalog books in some way has been present since books were invented. A good catalog enables people to know which books a library has, and where to find them, and many catalogs contain additional information which could be assistance to scholars. Early library catalogs were kept on scrolls or in ledgers, and they were often printed and distributed so that distant scholars could know which books a library had.

The concept of the card catalog was introduced in the 1800s, and it was a great help to scholars. Card catalogs can be configured in a number of ways, and their organization makes it easy to add or remove books, and to find books. In a card catalog, every time a new book enters a library, a card is created for it, with information like the title, author's name, subject, and location of the book.

There are a number of ways to set up a card catalog. For example, a dictionary catalog lists every single book in a library in alphabetical order, so in order to find a book, someone must know what the title is. An author catalog is organized alphabetically by author's name, while a keyword catalog is organized by subject. It is also possible to find systematic card catalogs, also called classified catalogs, which list the books according to the library's categorization system.

As anyone who has used a card catalog knows, it can be a challenge to use until one gets the hang of it, especially in a new library. Many patrons were forced to call on the librarian for help while navigating a difficult card catalog, but once one became acquainted with the system, the card catalog was a huge help, allowing patrons to quickly identify the books they wanted and find them on the shelves.

Computerized catalogs are the norm at most libraries today, because they have a number of obvious advantages over card catalogs, including space efficiency, searchability with multiple parameters, and the ability to easily update with realtime data.