PFS is a two-channel Michelson interferometer operating in the infrared wavelengths between 1.25 and 45 micrometers . The instrument is mainly devoted to the study of the Martian atmosphere. The principal goals are the measurement of the atmospheric temperature and pressure, atmospheric constituents, aerosol and clouds, ground pressure for surface topography, and optical and thermophysical properties of the Martian soil. PFS will fly on the Mars 94 spacecraft which should be launched in 1994 and reach the planet in 1995. Essentially it consists of two different interferometers located in the same box which is divided in two parts. An edge filter placed on the PFS entrance is used to separate the spectral range into two parts. The reason for that is the different optical materials which have to be used in each spectral range. The optical layout of the experiment is very compact. Cubic mirrors are mounted on an L-structure pivoted on a stepping motor. The stepping motor moves the mechanics and permits the optical path difference between the arms to be varied. Each interferometer operates in a different spectral range between 1.25 - 4.8 micrometers (8000 - 2083 cm-1) and 6 - 45 micrometers (1666 - 220 cm-1), respectively. The spectral resolution is 2 cm-1. The entrance aperture area is 30 cm2 per channel and the field of view (FOV) 2 and 4 degs. Every measurement lasts about 4 s and the respective resolving power is 4166 and 1041. The time and, therefore, the relative optical path difference for the measurement of every point of the interferogram is given by a monochromatic reference channel at 1.2 micrometers which uses a laser diode as a source. The interferograms are double sided and have 16384 and 4096 points, respectively, corresponding to spectra of 6250 and 1823 points.