Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a disturbance that affects an electrical circuit due to either electromagnetic conduction or electromagnetic radiation emitted from an external source.  The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit, the Sun or the Northern Lights.
EMI can be intentionally used for radio jamming, as in some forms of electronic warfare, or can occur unintentionally, as a result of spurious emissions for example through intermodulation products, and the like. It frequently affects the reception of AM radio in urban areas. It can also affect cell phone, FM radio and television reception, although to a lesser extent.
The International Special Committee Radio Interference sets standards for radiated & conducted electromagnetic interference.
EMI in integrated circuits
Main article: Electromagnetic compatibility
Integrated circuits are often a source of EMI, but they must usually couple their energy to larger objects such as heatsinks, circuit board planes and cables to radiate significantly.
On integrated circuits, important means of reducing EMI are: the use of bypass or "decoupling" capacitors on each active device (connected across the power supply, as close to the device as possible), rise time control of high-speed signals using series resistors, and VCC filtering. Shielding is usually a last resort after other techniques have failed, because of the added expense of shielding components such as conductive gaskets.
The efficiency of the radiation depends on the height above the ground plane or power plane (at RF one is as good as the other) and the length of the conductor in relation to the wavelength of the signal component (fundamental frequency, harmonic or transient (overshoot, undershoot or ringing)). At lower frequencies, such as 133 MHz, radiation is almost exclusively via I/O cables; RF noise gets onto the power planes and is coupled to the line drivers via the VCC and ground pins. The RF is then coupled to the cable through the line driver as common-mode noise. Since the noise is common-mode, shielding has very little effect, even with differential pairs. The RF energy is capacitive coupled from the signal pair to the shield and the shield itself does the radiating. One cure for this is to use a braid-breaker or choke to reduce the common-mode signal.
At higher frequencies, usually above 500 MHz, traces get electrically longer and higher above the plane. Two techniques are used at these frequencies: wave shaping with series resistors and embedding the traces between the two planes. If all these measures still leave too much EMI, shielding such as RF gaskets and copper tape can be used. Most digital equipment is designed with metal, or conductive-coated plastic, cases.
RF Immunity and testing
Integrated Circuits tend to demodulate high frequency carrier signals commonly found in regular environment due presence of cell phones. These ICs demodulate the high frequency cell phone carrier (e.g GSM850 and GSM1900, GSM900 and GSM1800) and produce low frequency (e.g 217Hz) demodulated signals. . This demodulation manifests itself into unwanted audible buzz in audio appliances such as microphone amplifier, speaker amplifier, car radio, telephones etc. Adding on-board EMI filters or special layout techniques help in bypassing EMI or improving RF immunity. Some ICs are designed (E.g LMV831-LMV834, MAX9724) to have integrated RF filters and/or special design which prevent demodulation of high frequency carrier. These ICs are also subjected to tests for measuring its capability to reject RF.
Designers often need to carry out special tests for testing the RF immunity of the parts to be used in the system. These tests are usually carried inside a special anechoic chamber with a controlled RF environment where the test vectors produce RF field similar to that produced in actual environment.