Terminal Access Controller Access-Control System (TACACS, /ˈtækæks/) refers to a family of related protocols handling remote authentication and related services for network access control through a centralized server. The original TACACS protocol, which dates back to 1984, was used for communicating with an authentication server, common in older UNIX networks including but not limited to the ARPANET, MILNET and BBNNET. It spawned related protocols:

TACACS was originally developed in 1984 by BBN, later known as BBN Technologies, for administration of ARPANET and MILNET, which ran unclassified network traffic for DARPA at the time and would later evolve into the U.S. Department of Defense's NIPRNet. Originally designed as a means to automate authentication – allowing someone who was already logged into one host in the network to connect to another on the same network without needing to re-authenticate – it was first formally described by BBN's Brian Anderson TAC Access Control System Protocols, BBN Tech Memo CC-0045 with minor TELNET double login avoidance change in December 1984 in IETF RFC 927. Cisco Systems began supporting TACACS in its networking products in the late 1980s, eventually adding several extensions to the protocol. In 1990, Cisco's extensions on top of TACACS became a proprietary protocol called Extended TACACS (XTACACS). Although TACACS and XTACACS are not open standards, Craig Finseth of the University of Minnesota, with Cisco's assistance, published a description of the protocols in 1993 as IETF RFC 1492 for informational purposes.

TACACS is defined in RFC 1492, and uses (either TCP or UDP) port 49 by default. TACACS allows a client to accept a username and password and send a query to a TACACS authentication server, sometimes called a TACACS daemon. It determines whether to accept or deny the authentication request and sends a response back. The TIP (routing node accepting dial-up line connections, which the user would normally want to log in into) would then allow access or not, based upon the response. In this way, the process of making the decision is "opened up" and the algorithms and data used to make the decision are under the complete control of whoever is running the TACACS daemon.

Extended TACACS (XTACACS) extends the TACACS protocol with additional functionality. It also separates the authentication, authorization, and accounting (AAA) functions out into separate processes, allowing them to be handled by separate servers and technologies.

TACACS+ is a Cisco designed extension to TACACS that is described in RFC 8907. TACACS+ includes a mechanism that can be used to obfuscate the body of each packet, while leaving the header clear-text. Moreover, it provides granular control in the form of command-by-command authorization.

TACACS+ has generally replaced TACACS and XTACACS in more recently built or updated networks. TACACS+ is an entirely new protocol which is not compatible with its predecessors, TACACS and XTACACS.

There are a number of differences between the two protocols which make them substantially different in normal usage.

TACACS+ can only use TCP, while RADIUS normally operates over UDP, but can also use TCP (RFC6613), and for additional security, TLS (RFC 6614) and DTLS (RFC7360).