SIM Application Toolkit (commonly referred to as STK) is a standard of the GSM system which enables the Subscriber Identity Module (SIM)

Subscriber Identity Module

A subscriber identity module or subscriber identification module is an integrated circuit that securely stores the International Mobile Subscriber Identity and the related key used to identify and authenticate subscriber on mobile telephony devices .A SIM is held on a removable SIM card, which...

 to initiate actions which can be used for various value-added service

Value-added service

A value-added service is popular as a telecommunications industry term for non-core services, or in short, all services beyond standard voice calls and fax transmissions. However, it can be used in any service industry, for services available at little or no cost, to promote their primary business...

s.

The SIM Application Toolkit consists of a set of commands programmed into the SIM which define how the SIM should interact directly with the outside world and initiates commands independently of the handset and the network. This enables the SIM to build up an interactive exchange between a network application and the end user and access, or control access to, the network. The SIM also gives commands to the handset such as displaying menus and/or asking for user input.

STK has been deployed by many mobile operators around the world for many applications, often where a menu-based approach is required, such as Mobile Banking

Mobile Banking

Mobile banking is a term used for performing balance checks, account transactions, payments, credit applications and other banking transactions through a mobile device such as a mobile phone or Personal Digital Assistant . The earliest mobile banking services were offered over SMS...

 and content browsing.
Designed as a single application environment, the STK can be started during the initial power up of the SIM card and is especially suited to low level applications with simple user interfaces.

In GSM 2G networks, the SIM Application Toolkit is defined by the GSM 11.14 standard released in 1995.
From release 4 onwards, GSM 11.14 was replaced by 3GPP 31.111 which also includes the specifications of the USIM Application Toolkit (USAT) for 3G networks.

Advantages

• Some manufacturers claim that STK enables higher levels of security through identity verification and encryption, which are necessary for secure electronic commerce.
• STK has been deployed on the largest number of mobile devices.

Limitations

• Updating STK applications and menus stored on the SIM can be difficult after the customer takes delivery of the SIM
  Subscriber Identity Module
  A subscriber identity module or subscriber identification module is an integrated circuit that securely stores the International Mobile Subscriber Identity and the related key used to identify and authenticate subscriber on mobile telephony devices .A SIM is held on a removable SIM card, which...
  
  . To deliver updates, either the SIM must be returned and exchanged for a new one (which can be costly and inconvenient) or the application updates must delivered over-the-air (OTA)
  Over-the-air programming
  Over-the-air programming refers to various methods of distributing new software updates or configuration settings to devices like cellphones and set-top boxes...
  
   using specialized, optional SIM features. Mobile Network Operator
  Mobile network operator
  A mobile network operator , also known as mobile phone operator , carrier service provider , wireless service provider, wireless carrier, or cellular company, or mobile network carrier is a telephone company that provides services for mobile phone subscribers.One essential...
  
  s can now , for example, deliver updated STK application menus by sending a secure SMS
  SMS
  SMS is a form of text messaging communication on phones and mobile phones. The terms SMS or sms may also refer to:- Computer hardware :...
  
   to handsets that include a SIMalliance Toolbox (S@T) compliant wireless internet browser (WIB). When using a Bearer Independent Protocol-compliant (BIP) SIM card in a BIP-compliant handset, the updates can be delivered very quickly as well (depending upon the network connectivit available to and supported by the handset, i.e. GPRS/3G
  3G
  3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 specifications by the International Telecommunication Union...
  
   speed). It might also be possible to change the menu of wireless internet gateway (WIG)-based STK applications. The update limitations hinder the number and frequency of STK application deployments.
• STK has essentially no support for multimedia
  Multimedia
  Multimedia is media and content that uses a combination of different content forms. The term can be used as a noun or as an adjective describing a medium as having multiple content forms. The term is used in contrast to media which use only rudimentary computer display such as text-only, or...
  
  , only basic pictures.
• The STK technology has limited independent development support available.

STK and CAT in 3G

USIM Application Toolkit (USAT) is the equivalent of STK for 3G

3G

3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 specifications by the International Telecommunication Union...

networks.

Card Application Toolkit (CAT) is now used as the more generic method used describing the UICC's more extensive STK features.