Plasmas are capable of supporting a number of oscillatory motions. Besides wave motion, special ``positive feedback'' modes can arise - so-called instabilities. These originate as small perturbations in the plasma which grow to large amplitudes. An important consideration when dealing with instabilities is the origin of the instability, its growth rate and the threshold factors for the instability to occur. In fact, so many processes contribute to the generation of structures in the plasma density and velocity field of the high-latitude ionosphere that only a few important mechanisms will be discussed. The importance of these instabilities for the SuperDARN radar is simple: they produce the irregularities which scatter the radio waves. An extended discussion about high-latitude F-region irregularities can be found in a review by Tsunoda [Tsu88].