The models of unstable nucleate and film boiling are suggested. The model of bulk boiling takes into account the processes of heat transfer from an external source, nucleation, bubble growth and withdrawal of bubbles from the metastable zone. The surface of neutral stability for stationary boiling regimes is obtained and bifurcation analysis of the threshold of instability is carried out. The amplitude and frequency of non-linear self-oscillating regimes are calculated. The analysis of unstable boiling on a heating surface is based on the model of conjugate heat transfer problems in a plate of finite thickness and in a boundary layer. The conditions of transfer from pulsating nucleate boiling to the dry patch formed during boiling with repeated rewetting are determined. Theoretical results are compared with available experimental data.