By electromagnetic compatibility is understood the ability of an electrical equipment to function in its electromagnetical environment satisfactorily, without unpermissibly influencing this environment to which also other equipments belong.

A component or an apparate is therfore electromagnetically compatible, if in the function as a transmitter a tolerable emission with reference to the interference emission is not exceeded and as a receiver a satisfactory immission, i.e. immunity is guaranteed. Electromagnetic influences may lead to reduction of functions or functional disturbances. Click interferences in radio reception by switching processes in the household for instance, are reversible interferences, whilst destruction of electronic components on circuit boards are called irreversible disturbances. Electromagnetic influences in general are defined as effects of electromagnetic factors on circuitry, apparates, systems or creatures. 
 
The reasons for EMC
The growing EMC-problems may be attributed to increased employment of microelectronics in automation systems, automobiles, household- and telecommunication-equipment. This becomes more vulnerable against interference coupling, since ever more sensitive semi-conductor types with ever lower logic levels are used, higher grades of networking favour more coupling paths and the faster growing systems are no longer able to integrate transient interferences. Furthermore also the general electromagnetic environmental loading by h.f. news transmission, e.g. in form of mobile phones, has increased. Since the geometrical dimensions of circuits come more frequently into the magnitude of the used or predominant wavelenght, this results in unwanted radiations, respectively, undesired coupling of electromagnetic fields.

In order to avoid functional failures of apparates and systems, in particular technology with relevance to safety, the legislator has determined norms which meanwhile shall be complied by all electrical and electronic apparates coming into the market. The test samples shall neither exceed the limit values fixed for immunity- nor for interference emission.

The EMC-Law
For reduction of barriers to trade within the member states of the European Union (EU) a few years ago there had come into force a number of important changes. In scope of this hamonisation the EU declared the EMC in May 1989 as a general protective target and passed appropriate EC-Directives, which had to be transposed by the member states into their national law. The German Lower House for example, followed this obligation already on 13.11.1992 and decided on base of the defined EC protective requirements the first law for securing  the EMC of apparates. The (German) Law for Electromagnetic Compatibility (EMVG) contains besides the general guidelines also state-specific particularities and concrete administrative executive regulations. Technical details such as limit values or measurement procedures however are not be found here. For this the European EMC-norms ("EN"-norms= European Norms) are drawn on, the compliance of which according to the up-to-date status of  knowledge permits the assumption that the protective requirements of the EMVG are not violated.

Since 01.01.1996 there exists practically for all apparates of the electrical  / electronic field in Europe the legal duty for CE-marking (exceptions: amateur radio equipment, particular apparates for medical purposes and unique devices for needs of laboratory or exhibition). Since this time, within the European Economic Community (EEC: EU-member states + EFTA countries) no product may be marketed, which does not suffice the EC-Directive.

The CE-mark is not granted, but it is attached by the manufacturer/ importer on his own responsibility in agreement with all protective requirements. Here care has to be taken that an apparate consisting of several CE-marked units is not essentially CE-conform (altered ancillary conditions by the assembly / the cabling). The manufacturer / importer is obliged to subject his product to extensive measurements and investigations, the results of which shall be recorded in adequate reports. These reports shall be retained by the manufacturer / importer and procured on demand of the inspection authority.