The P25 Inter-RF Subsystem Interface (ISSI) is a standards based technology that is widely talked about among public safety communications interoperability. However, it is not frequently implemented or utilized in P25 radio systems across the Country. There are many myths and misconceptions of what the P25 ISSI can provide in terms of interoperability and connecting multiple radio systems. In this presentation, we will discuss what is required for the ISSI in the P25 standards, which vendors have completed the CAP testing to certify their ISSI's and features and functionality that you should use/avoid when connecting multiple P25 radio systems together.