A subscriber identity module is a removable smart card for mobile phones. SIM cards store the required information to identify the mobile device. It also contains data required for voice encryption to make listening in on calls almost impossible (except when the wireless carrier itself is doing the eavesdropping).

In this way the customer ID (and personal number) is tied to the SIM card and not to a certain mobile phone. This allows for a seamless interchange of the same SIM card between different GSM mobile phones.

SIM cards also serve as storage for SMS messages and the user's contacts. Current SIM cards can store up to 250 name/number pairs and up to 50 SMS text messages.

The SIM card cannot store multiple numbers per contact or other more complex information. This means that if you copy your contacts info from the phone memory to the SIM memory, contacts get broken up into as many entries as there are numbers for each individual contact and discards the other information.

All GSM phones and most iDEN phones require a SIM card to operate.

There are certain types of phones (CDMA, TDMA, AMPS) that do not use a SIM. Instead, the required data is programmed directly into the phone.

Replaceable SIM cards come in four standard sizes:

• Full-size (85.6mm × 53.98mm × 0.76 mm)
• Mini-SIM (25mm x 15mm x 0.76mm)
• Micro-SIM (15mm x 12mm x 0.76mm)
• Nano-SIM (12.3mm × 8.8mm × 0.67mm)

The eSIM or Embedded SIM comes in one size:

• eSIM (6mm x 5mm x <1mm>

***The diagram above shows how the SIM card has reduced in size over the years