[en] Spiral density waves are amplified because of a transfer of angular momentum from the rapidly rotating disk to the slowly rotating bulge-halo subsystem of galaxies. This transfer is more efficient, thus amplification is higher, for waves of more open winding. Those waves are amplified whose radial positions are located within their respective corotation radii (where wave pattern frequency of rotation equals local rotation freqency in the disk). Although these waves possess significant group velocities, the emission processes at corotation prevent total outward leakage by redirecting amplified signals toward the inner regions where further amplification could occur.Provided that the bulge-halo subsystem comprises at least about 10 percent of the total mass in a galaxy, up to order-of-magnitude amplification (sometimes more) is possible for the waves driven by small barlike or oval distortions (section IVa). The processes at corotation have the additional advantage in that they allow tightly wrapped trailing spirals to be the result of such driving influences. On the other hand, maintenance of spiral structure might also occur as standing waves which grow from ''noise'' to finite amplitudes over the quasi-stationary timescale of several billion years. Faster growth is possible if the central or inner Lindblad resonance regions in galaxies are efficient in returning feedback signals to the corotation regions located farther out (even for the above-quoted bulge-halo masses; cf. section IVb)