IBM CICS (Customer Information Control System) is a family of mixed-language application servers that provide online transaction management and connectivity for applications on IBM mainframe systems under z/OS and z/VSE.

CICS family products are designed as middleware and support rapid, high-volume online transaction processing. A CICS transaction is a unit of processing initiated by a single request that may affect one or more objects. This processing is usually interactive (screen-oriented), but background transactions are possible.

CICS Transaction Server (CICS TS) sits at the head of the CICS family and provides services that extend or replace the functions of the operating system. These services can be more efficient than the generalized operating system services and also simpler for programmers to use, particularly with respect to communication with diverse terminal devices.

Applications developed for CICS may be written in a variety of programming languages and use CICS-supplied language extensions to interact with resources such as files, database connections, terminals, or to invoke functions such as web services. CICS manages the entire transaction such that if for any reason a part of the transaction fails all recoverable changes can be backed out.

While CICS TS has its highest profile among large financial institutions, such as banks and insurance companies, many Fortune 500 companies and government entities are reported to run CICS. Other, smaller enterprises can also run CICS TS and other CICS family products. CICS can regularly be found behind the scenes in, for example, bank-teller applications, ATM systems, industrial production control systems, insurance applications, and many other types of interactive applications.

Recent CICS TS enhancements include new capabilities to improve the developer experience, including the choice of APIs, frameworks, editors, and build tools, while at the same time providing updates in the key areas of security, resilience, and management. In earlier, recent CICS TS releases, support was provided for Web services and Java, event processing, Atom feeds, and RESTful interfaces.