  This short and opinionated review starts with a concept of quantum signals at
microwave frequencies and focuses on the principle of linear parametric
amplification. The amplification process arises from the dispersive
nonlinearity of Josephson junctions driven with appropriate tones. We discuss
two defining characteristics of these amplifiers: the number of modes receiving
the signal, idler and pump waves and the number of independent ports through
which these waves enter into the circuit.
