The subject of this volume is two-fold. First, it gathers typical polarization patterns occurring in nature. Second, it surveys the polarization-sensitive ani­ mals, the physiological mechanisms and biological functions of polarization sensitivity as weIl as the polarization-guided behaviour in animals. The monograph is prepared for biologists, physicists and meteorologists, espe­ cially for experts of atmospheric optics and animal vision, who wish to under­ stand and reveal the message hidden in polarization patterns of the optical environment not directly accessible to the human visual system, but measur­ able by polarimetry and perceived by many animals. Our volume is an attempt to build a bridge between these two physical and biological flelds. In Part I we introduce the reader to the elements of imaging polarimetry. This technique can be efflciently used, e. g. in atmospheric optics, remote sens­ ing and biology. In Part 11 we deal with typical polarization patterns of the natural optical environment. Sunrise/sunset, clear skies, cloudy skies, moonshine and total solar eclipses all mean quite different illumination conditions, wh ich also affect the spatial distribution and strength of celestial polarization. We pre­ sent the polarization patterns of the sky and its unpolarized (neutral) points under sunlit, moonlit, clear, cloudy and eclipsed conditions as a function of solar elevation. The polarization pattern of a rainbow is also shown. That part of the spectrum is derived in which perception of skylight polarization is optimal under partly cloudy skies.