A Subscriber Identity Module (SIM) is part of a removable smart card ICC (Integrated Circuit Card), also known as SIM Cards, for mobile, telephony devices (such as computers) and mobile phones. SIM cards securely store the service-subscriber key (IMSI) used to identify a subscriber. The SIM card allows users to change phones by simply removing the SIM card from one mobile phone and inserting it into another mobile phone or broadband telephony device.

SIM cards are available in two standard sizes. The first is the size of a credit card (85.60 mm × 53.98 mm x 0.76 mm). The newer, more popular miniature-version has a width of 25 mm, a height of 15 mm, and a thickness of 0.76 mm. However most SIM cards are supplied as a full-sized card with the smaller card held in place by a few plastic links and can be easily broken off to be used in a phone that uses the smaller SIM.

The first SIM Card was made in 1991, with Munich smart card maker Giesecke & Devrient selling the first 300 SIM cards to Finnish wireless network operator Elisa Oyj (formerly Radiolinja).

The exact format of the SSN differs according to the mobile operator, however the following is constant:

• The first 3 digits represents the Mobile Country Code (MCC).
• The next 2 digits represent the Mobile Network Code (MNC, i.e. the mobile operator).
• The next 12 digits is the number, from which some operators use the two left most to indicate the Home Location Register.
• The 20th digit then is a checksum.

The numbering of the ICC-ID is based on International Standard ISO/IEC 7812. The maximum length of the visible card number is 20 characters; 19 digits are preferred, but telecommunication network operators who are already issuing Phase 1 SIM cards with an identification number length of 20 digits may retain this length. The number is composed of the following subparts:

Issuer Identification number (max. 7 digits)

• Major Industry Identifier (MII), 2 digits, 89 for telecommunication purposes.
• country code, 1-3 digits, as defined by ITU-T recommendation E.164.
• issuer identifier, variable.

Individual account identification

• individual account identification number.
• parity check digit.

W-SIM is a SIM card which also integrates core cellular technology into the card itself.

A Virtual SIM is a mobile phone number provided by a wireless carrier which does not require a SIM Card to terminate phone calls on a user's mobile phone.