Polarization and directionality of Earth reflectances (POLDER) is an optical imaging radiometer developed by CNES for flight on NASA's ADEOS mission (launch date 1995). It is a wide field of view (50 degree(s)) and moderate spatial resolution (6 km) instrument, giving an almost daily coverage of the Earth. It gives access to the bidirectional reflectance and polarization distribution functions (BRDFs and BPDFs) by acquiring, for a given target, measurements in variable viewing configurations along the track of the satellite. These measurements are acquired in nine spectral bands of the visible-near infrared spectrum, and at three polarization angles at 443, 665, and 865 nm. Its main scientific objectives are: (1) tropospheric aerosols -- determination of the characteristics of the aerosols; (2) ocean color -- accurate determination of the sea surface reflectances, due in particular to improved capabilities of atmospheric corrections; (3) land surface -- determination of surface BRDFs and improvement of the correction of surface bidirectional effects and atmospheric effects on vegetation indices; (4) Earth radiation budget and clouds -- determination of clouds BRDF and classification of clouds according to bidirectional radiative properties, but also on cloud altitude and phase. The objective of the paper is to present and discuss these mission objectives.