The Sun’s radiation becomes linearly polarized by coherent scattering processes in the solar atmosphere. With a novel polarimetry system that achieves a precision of 1e-5 in the degree of polarization, the previously largely unexplored territory of scattering physics on the Sun is now fully accessible. The observations reveal a polarized spectrum that looks very different as compared with the ordinary, unpolarized solar spectrum but has an astounding wealth of spectral structures. It is therefore refered to as the ``second solar spectrum’’. In the present paper we show how the second solar spectrum is governed by different physical processes, which provide new diagnostic opportunities and tools that are complementary to those of the ordinary intensity spectrum. We illustrate the effects of quantum interferences and hyperfine structure, isotope abundances, partial frequency redistribution, molecular contributions, and magnetic canopies. Also shown are polarization features, for which the underlying physics has not yet been identified.