The High Level Architecture (HLA) is the new generation architecture of the Distributed Interactive Simulation (DIS). Its purpose is to facilitate interoperability among simulations and promote reuse of simulations and their components. RTI software is the core to integrate and support the HLA simulations. It is important to emphasize the interoperability of the RTI software. The RTI is usually based on peer-to-peer model of data exchange, and it adopts the distributing integration frame in client/server mode, which accords with the basic requirements of current software interoperability. The RTI software solves three essential problems to support the interoperability: 1) Functional Matching, 2) Interface Matching, and 3) Executing Supporting. According to the differences in handling with the interoperation between simulations, the approaches those make a simulation HLA interoperable can be induced to two methods: one is the Interface-Standardization-Based Method, the other is the Gateway-Based Method. The Interface-Standardization-Based method has the advantage of simple and useful to develop a new federation, and the Gateway-Based method is efficient to reduce the work of integrating legacy simulations.