This chapter is devoted to the most important problem for us in this book: the various radio electronic amplifiers. First (Sect. 3.1), we describe the most important characteristics of amplifying structures; discuss the destination, peculiarities, and general characteristics of amplifiers; the quality parameters of amplifiers; the types of amplitude-frequency characteristics; the pass-band structure; linear, nonlinear, and intermodulation distortions; the gain of multistage amplifiers; and the amplitude characteristics. After we describe the noise parameters, the noise factor and the noise temperature are obtained. Section 3.2 is devoted to transistor amplifiers beginning from the general models of active components: linear models of bipolar transistors and field-effect transistors (FETs), including FETs with Schottky barriers (MESFETs), metal oxide semiconductors (MOSs), and heterostructure transistors. We discuss resistor amplifying stages for bipolar and field-effect transistors and methods for operating point stabilization, including the introduction of feedback, circuits with common emitters (sources in FETs), common (drains), and common bases (gates). We consider differential amplifiers, DC amplifiers (op-amps), amplifiers with symmetric and nonsymmetric input and output, cascades with current mirrors, voltage-controlled amplifiers, generators of stable current, resonance (tuned) amplifiers, RF amplifiers, IF amplifiers, low-noise amplifiers on bipolar transistors, and FETs. Methods for improving the amplifier stability are described in detail, including the use of cascode LNAs and balanced LNAs. Then, we discuss amplifiers with digital modulation signals and microwave amplifiers, including those with strip transmission lines. Regenerative amplifiers of different types are described in Sect. 3.3. In the appendix to this chapter, we describe the calculation of the low-noise amplifier on the MOS transistor.