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The principles of parametric amplification are described through the use of simple low‐frequency electrical circuits and their mechanical analogs. Extension of these principles to low‐noise amplification at microwave frequencies is made in a qualitative way which emphasizes the physical processes involved. Amplification is only achieved when energy is transferred from a microwave oscillator or pump to the signal frequency, and it is shown why such a transfer may take place in circuits exhibiting variable capacitance or inductance but not in circuits where only a resistance is varied. Major noise sources are discussed and the special role of the image or idler frequency is noted. Finally, experimental data on four different types of parametric amplifier (3 using semiconductor diodes and 1 using ferrite) are given.