Nitrogen dioxide (N0 ) is notorious for its complex visible spectrum and has 2 frustrated many chemists and physicists. Despite their intense investigations the molecule still resists complete analysis of its spectrum. Because of the high density of lines, it is apparent that high-resolution measurements are essential to a thorough understanding of the spectrum. The major part of this book consists of an atlas, described in Chap. 2, of the absorption spectrum and the Stark modulation spectrum of N0 mea 2 sured using a cw dye laser. The narrow spectral width as well as the wide scan range of the dye laser made it possible to record the spectra contin uously over a wide range with Doppler-limited resolution. In the spectral range 16751-17885 em-I, about 13300 peaks have been observed in the absorption spectrum and their wave numbers, calibrated against Ar lines, are listed in the tables. The Stark modulation spectrum also shown provides useful information about the energy level structures complementary to that obtained from the absorption spectrum.