The infrared spectral range (lambda= [1-100] micro-m) covers the main roto-vibrational 
bands of several molecules being present in planetary atmospheres. 
Namely, key species for chemical cycles such as CO2, H2O and CH4 strongly interact 
with the radiation field in the IR domain. The same spectral region hosts distinctive 
signatures of aerosols, a ubiquitous presence in planets with a substantial atmospheres. 
Consequently, despite the technical difficulties related to its implementation, 
IR spectroscopy has represented the choice investigation tool for the remote sensing of 
planetary environments since the beginning of space exploration.
This talk shortly illustrates the physical concepts behind these investigations starting 
from a general form of the radiative transfer equation appropriate for planetary atmospheres. 
Main interpretative techniques are mentioned, while attention is paid to the ambiguities 
in data interpretation related to the non uniqueness of solution for the ill-posed inverse 
problems.
A synopsis of past atmospheric studies is provided, with data examples of from Mariner 9 
(Mars), Venera 15 (Venus) and Voyager (giant planets) payloads. The implementation of 
spectro-imagining instruments since the beginning of the 1990s marked a decisive step 
forward in the planetary investigations, with Galileo and Cassini missions. 
In very recent years, a renewed interest toward terrestrial planets leaded to the 
integration of high-spectral/high-spatial resolution payloads. 
Examples in this sense are the ESA missions Mars Express and Venus Express, 
whose results are extensively reviewed here.