EMI (Electromagnetic Interference), also known as RFI (Radio Frequency Interference), is unwanted electromagnetic energy polluting the environment. Its propagation via radiation and power conduction over system signal and power lines can affect the operating of electrical equipment around the source. EMC (Electromagnetic Compatibility) is the ability of a system to function reliably in the presence of significant levels of EMI and at the same time to limit its internally generated EMI to avoid interference with the operation of other systems around it.

EMI effects are not only a nuisance but can also be destructive. Effects can range from annoying picture disturbances on TV sets to an aeroplane crash due to an EMI related flight control failure. EMI can not be completely eliminated but can be attenuated to safe levels recommended by the International Special Committee on Radio Interference (CISPR). Most countries, especially the EU and US, have regulatory bodies that set EMC regulations based on CISPR recommendations. In Europe the national regulatory bodies of member countries operate within the scope of the EMC Directive issued by the European Commission, while in the US, the Federal Communications Commission (FCC) is the guiding body.