Secure Digital or (SD) is a non-volatile memory card format for use in portable devices. The Secure Digital standard is maintained by the SD Card Association (SDA). SD technologies have been implemented in more than 400 brands across dozens of product categories and more than 8,000 models.

The Secure Digital format includes four card families available in three different form factors. The four families are the original, Standard-Capacity (SDSC), the High-Capacity (SDHC), the eXtended-Capacity (SDXC), and the SDIO, which combines input/output functions with data storage. The three form factors are the original size, the "mini" size, and the "micro" size (see illustration). There are many combinations of form factors and device families.

Electrically passive adapters allow the use of a smaller card in a host device built to hold a larger card.

Host devices that comply with newer versions of the specification provide backward compatibility and accept older SD cards, but older host devices do not recognize newer cards. The SDA uses several trademarked logos to enforce compliance with its specifications and assure users of compatibility. This article explains several factors that can prevent the use of a newer SD card: