At the resonance, such transformers offer very high voltage conversion factors up to 1000 with voltages of 5 – 15 kV. Electric discharges produced in the gas from the high voltage side of the piezoelectric transformer have properties found also in the corona discharges and in the dielectric barrier discharges. While the former mode occurs when the high voltage side of the piezoelectric transformer is operated far from the electric grounds, the latter mode occurs when it is operated close to the electric grounds separated by a dielectric. Near the open electric grounds, the piezoelectric transformer produces periodic sparks. Transition to the electric arc does not occur because of the limited power of the transformer. The typical power of such transformers is of the order of 10 W. The efficiency of the plasma generation reaches 90%, while the remaining 10% of the power is lost due to mechanical heating of the piezoelectric transformer. Due to low electric currents, typical for the dielectric barrier and the corona discharges, the piezoelectric direct discharge produces a non-equilibrium plasma. This means that its constituent electrons, ions and the neutral gas particles have different kinetic energy distributions. Temperature of the neutral gas within the plasma volume remains lower than 50 C. At the same time, the electrons and the ions reach energies of 1 – 10 eV. This is 300 – 3000 times higher than the average energy of the neutral gas particles. The densities of the electrons and the ions reach 10−16 – 10−14 m−3. Since most of the plasma volume consists of the cold neutral gas, the plasma is cold. However, the very energetic electrons and ions excite atoms and molecules producing large amounts of short-lived chemical species, making this plasma chemically very active. Properties of the piezoelectric direct discharge plasmas enable a large spectrum of applications in medical technology, microbiology and clinical research. Typical industrial applications include ultra-fine cleaning and plasma activation of metal, ceramic, glass and plastic surfaces. Such plasma processing increases the surface energy improving the surface wettability and adhesion. The latter increases the quality of the subsequent printing or gluing. Very compact dimensions of the PDD plasma generator further broaden the sphere of possible applications to compact devices for laboratory work, hand-held applications, ozone generators, and even consumer products.