The study of the interaction of matter and electromagnetic radiation in the microwave region of the spectrum. Microwaves are loosely defined as electromagnetic radiation with wavelengths between about 1 mm and 30 cm or frequencies between 1 and 300 GHz. The wavelengths are comparable to the dimensions of experimental apparatus. Experimental techniques make use of ideas from radio-frequency spectroscopy where wavelengths greatly exceed the dimensions of the apparatus, and also techniques from optics where wavelengths are much smaller than the size of the apparatus.