A SIM or subscriber identity module is a smart card inside the GSM cellular phone that stores International Mobile Subscriber Identity (IMSI) to identify and authenticate the user on mobile telephony devices. SIM is a small card that contains programming and information such as phonebook entries. The major advantage of GSM architecture is that the SIM can be moved from one mobile station to another.

Depending on its size, the SIM can be classified as
• Full-size
• Mini-SIM
• Micro-SIM
• Nano-SIM
• Embedded-SIM
SIM cards are necessary in all cell phones for connecting to any particular network. The flash memory integrated in SIM cards also works as a memory card for storing the information such as phonebook contacts, messages, and others. Most of the currently available SIM cards come with the capacity of 32 or 64 KB of storage for holding the personal identity information, and other data. With the incorporation of multimedia functions in mobile phones, the need for SIM cards with more memory storage capacity is increasing. Hence, SIM card manufacturers are engaged in developing SIM cards with superior storage capacities, allowing the storage of all kind of digital files.

SIM card is proving as an alternative solution to the memory cards; however, they are not compatible with other devices except cell phones. SIM cards act as an in-built storage as they are not that portable. Additionally, there are very few card readers that support the format of SIM cards, which is limiting the use of SIM cards as memory cards.

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Some of the key players engaged in manufacturing of SIM cards include Datang Microelectronics Technology Co., Ltd., Ebtikar Card Systems, Gemalto NV, Giesecke & Devrient, Eastcompeace Smart Card Co., Ltd., KEBTechnology, Eastcompeace Smart Card Co., Ltd., Solacia, TianYu Inc., and Watchdata Technologies Pte Ltd., Morpho, Oberthur Technologies, and others.