Microwave processing shows promise as a material synthesis method for metastable phases and contributes to achieve energy-efficient processes. This paper outlines the synthesis of functional ceramics under microwave electromagnetic fields in terms of solid-solid and solid-gas reactions. Microwave-assisted synthesis of functional materials, such as oxides and nitrides has been shown to save energy and reduce reaction times compared to conventional methods. Local selective heating under microwave irradiation has been experimentally demonstrated to generate significant temperature differences and microwave-induced solid-gas reactions involve a mechanism of plasma generation from gas-phase excitation, supported by emission spectroscopy.