First, the dominant radiation modules and EMI coupling paths in an explicit optical module are analyzed using simulation and measurement techniques. Correspondingly, practical mitigation approaches are proposed to suppress the radiation in real product applications. To predict the EMI level of a router-like system, the EMI of individual mo ules needs to. Abstract The multitude of Electrical/Optical interfaces, such as QSFP or OSFP modules, lead to the accumulation of EMI in larger Switches and Routers. Levels far above the level of an individual module can be reached, possibly causing unacceptable levels of EMI from a system filled with many. Electromagnetic interference (EMI) is becoming more troublesome in modern electronic systems due to the continuous increase of communication data rates. This chapter reviews some new methodologies for high-frequency EMI diagnostics in recent researches.