CAPI stands for Common ISDN Application Interface and was developed with the aim of creating a uniform interface between application software and the ISDN end-devices (e.g. ISDN card).

In principal, CAPI enables the use of any ISDN card supporting CAPI together with any ISDN application software which also supports CAPI. At the forefront in the development of CAPI were German companies from the ISDN sector and Deutsche Telekom.

There are at present two versions of CAPI in use. CAPI versions 1.0 and 1.1 have been designed for PC-based ISDN solutions. They are supported by nearly all PC-based ISDN cards. The essential improvement of the latest version CAPI 2.0 provides support for any desired platform, making it also suitable for UNIX environments. Platform independence is one reason why Windows NT, uses CAPI 2.0 in its Remote Access Services (RAS).