This chapter examines system-level electromagnetic compatibility (EMC), although many of the topics are equally applicable to subsystems and equipment. For example, antenna-to-antenna or antenna-to-cable coupling can occur within a subsystem as well as between systems. For the system-level main filter, the source of noise on the load side of the filter is a composite of the individual equipment noise sources, as is the total load impedance presented by the equipment. The analysis of the performance of the main system-level filter is likely to be more complex than that required for an equipment filter and may be practical only by use of a circuit modeling program such as SPICE. Margins shall be provided based on system operational performance requirements, tolerances in system hardware, and uncertainties involved in verification of system-level design requirements. Intersystem EMC covers compatibility with, but is not limited to, EMEs from like platforms friendly emitters, and hostile emitters.