Modern view on the nature of spiral structure of galaxies

The current state of the Lin-Shu density wave theory is discussed according to modern observational data. Much attention is paid to the problem of wave excitation and to the response of the interstellar gas to the wave gravitational potential. It is noticed that the major predictions of the theory - the galactic shocks, the spiral velocity field of stars, and the age gradient across the spiral arms - have become nowadays the fundamental observational facts so the density wave theory is now out of competition among alternative theories. The nature of flocculent spirals as it believed being connected with the induced star formation in differentially rotating galactic disk rather than with the density waves is also discussed