In-Service Engineering, or ISE, is the term used to describe support for systems already installed on ships - thus, while they are in service- in contrast to future systems that are still being developed.

In-Service Engineering is a very broad function that includes the installation of systems on ships, certification that the systems perform as designed, and the training and qualification of the Sailors that operate and maintain the systems. It also entails ensuring that the logistics support required (such as training plans, maintenance procedures, spare parts, etc.) is in place when the system is installed.

The organization designated as the lead for implementing In-Service Engineering is called the ISE Agent.  The Fleet Support Team (FST) is a term that the Navy, particularly NAVAIR, uses for in service engineering.