  Slow light in photonic crystals and other periodic structures is associated
with stationary points of the photonic dispersion relation, where the group
velocity of light vanishes. We show that in certain cases, the vanishing group
velocity is accompanied by the so-called frozen mode regime, when the incident
light can be completely converted into the slow mode with huge diverging
amplitude. The frozen mode regime is a qualitatively new wave phenomenon -- it
does not reduce to any known electromagnetic resonance. Formally, the frozen
mode regime is not a resonance, in a sense that it is not particularly
sensitive to the size and shape of the photonic crystal. The frozen mode regime
is more robust and powerful, compared to any known slow-wave resonance. It has
much higher tolerance to absorption and structural imperfections.
