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The well-known theory of fourpoles only comprises passive fourpoles and active fourpoles with internal sources of sinusoidal currents or voltages of defined frequencies. This theory is now completed for fourpoles with internal noise sources. Simple equivalent circuits are derived for such networks. They consist of the original but noise-free fourpole cascaded with a preceding noise fourpole in which all noise-sources are concentrated. The latter contains the equivalent noise conductance G., the equivalent noise resistance Rn and the complex correlation admittance Ycor. With these quantities the noise behavior of any desired fourpole can be described sufficiently. In particular it is possible to calculate the noise figure F and its dependence on the matching conditions to the signal source of a single fourpole or a group of cascaded fourpoles. The methods of experimental determination of the elements of the noise fourpoles are discussed. The same theory is also useful for mixer-circuits as well as fortraveling-wave tubes and transistors, as application results are given for grid controlled electron tubes.