Our talk deals with the plasma oscillations in two-dimensional heterostructures like high-electron-mobility transistors. In such structures with the channel length of about one micrometer and less, the frequencies of the plasma oscillations are in the terahertz range. The two-dimensional electron gas in the structures can act as a resonant cavity. Due to a high electron mobility the quality factor of this resonant "cavity' can be rather high. This opens up wide prospect to use the plasma effect in the structures in question in different terahertz devices, in particular, sources of terahertz radiation. We will start from a brief review of the plasma effects in low-dimensional heterostructures. Next we will consider the plasma instabilities and self-excitation of plasma oscillations in two-dimensional heterostructures, detection and mixing of terahertz radiation associated with inherent nonlinearities, and photomixing involving the excitation of the plasma oscillations by optical signals. Afterwards the experimental results and feasibility of the effective device creation will be discussed.