This paper reviews our current knowledge of the spectra of planets, from the centimeter to the submillimeter range, both from an observational and a theoretical point of view. In the case of Venus, due to its very thick CO{sub}2 atmosphere, the spectrum is dominated by the pressure-induced CO{sub}2 absorption, with some other contributions from minor constituents (SO{sub}2, H{sub}2O). In the case of Mars, which has a very tenuous atmosphere, the microwave spectrum is composed of discrete narrow molecular lines of CO, H{sub}2O and their isotopes, superimposed over a continuum defined by the surface emissivity and temperature. The microwave spectra of the giant planets probe their tropospheres. They are dominated by collision-induced absorption, mostly due to H{sub}2-H{sub}2 and H{sub}2-He collisions, with, in the case of Jupiter and Saturn, additional features due to NH{sub}3 and PH{sub}3. For all cases, observed and synthetic spectra are described, and the various sources of uncertainty are discussed.
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