The aim of this review is to show the occurrence of non-thermal effects in organic synthesis. The effect of microwave irradiation is a consequence of the interaction of radiation with matter and a combination of thermal and non-thermal effects. Thermal effects are well-described and arise from the heating rate, sometimes non accessible by conventional heating, superheating, "hot spots" and the selective absorption of radiation by polar substances. The existence of non-thermal effects of the highly polarized radiation is still a controversial topic, and one usually masked by thermal effects. Separation and identification of thermal and non-thermal effects is a complex matter, but essential to the study of non-thermal effects. Some predictive models have also been described.