The ISODE software, originally perhaps intended to be an ISO Development Environment, was an implementation of the OSI upper layer protocols, from transport layer to application layer, which was widely used in the Internet research community to experiment with implementation and deployment of OSI during the late 1980s and early 1990s.

While the ISODE implementation could be configured to use one of several X.25 (CONS) or connectionless lower layer protocols, many ISODE deployments were based on [RFC 1006], the implementation of OSI transport protocol TP0 as a layer atop TCP, in order to use IP-based networks which were becoming increasing common.

ISODE formed the basis for popular implementations of X.400 (PP) and X.500 (QUIPU), and the approaches for providing a simplified directory lookup protocol that was easier to implement in a client than X.500's Directory Access Protocol led to the development of the Lightweight Directory Access Protocol.

Implementations of FTAM and VT were included in ISODE, and implementations of X.400 P7 (PPMS), TMN (OSIMIS) and other OSI protocols were also made atop ISODE.

Isode is also the name of the company founded by Steve Kille, one of the LDAP protocol authors to produce and market LDAP/X.500 directory servers and messaging systems based on the X.400 protocol.