We consider three groups of hydrodynamic instabilities of electron-hole plasmas in conducting solids, namely, helicoidal, two-stream, and overheating instabilities. The helicoidal instability is discussed in Chap. II and is due to the drift of electrons relative to the crystal lattice in constant electric and magnetic fields. It is manifested by the growth of transverse and longitudinal acoustic oscillations which interact with helicons. Particular attention is devoted to the explanation of the role of the magnetic field due to a constant current. In Chap. III, we give a summary of theoretical and experimental results on the interaction between a beam of electrons moving near the surface of a semiconductor and the associated electromagnetic waves. A detailed analysis is given of the size effect, i.e., the effect of the finite size of the specimen on the growth rates of two-stream instability. These growth rates exhibit a rapid rise in resonances. Chapter IV is concerned with the instability due to the heating of the electron gas by a constant electric field. Since the rate at which energy is transferred to the lattice is low, the static currentvoltage characteristic includes a falling segment (negative differential resistance). This leads to an instability of temperature perturbations and the associated electromagnetic waves. Particular attention is devoted to the assessment of conditions under which overheating instability has an oscillatory character.