Publisher Summary This chapter discusses the gunn-hilsum effect electronics. It presents a discussion of the operating principles and the characterization of short negative differential mobility (NDM) devices. It is seen that the material parameters of the device, the metal-semiconductor contact, the circuit, the space charge distribution, and the temperature distribution all affect device operation. All of these contributions manifest themselves in the DC-driven oscillator output, in the performance of small and large signal amplifiers, and in the noise properties of the device. There is now abundant experimental evidence supporting the fact that the space charge distribution within the NDM element is non-uniform and that the acceptance of this feature is the only way to reconcile the experiment with theory. Device technology is devoted to making better devices. Therefore, better control over the contact procedures and materials growth is always being sought. However, from the point of view of understanding the device operation, it is clear that the knowledge of the metal-semiconductor interface is still the weak line. Correlation of the experiment with analytical theory and large-signal computer simulation still requires an adequate low-resistance contact theory.