  Mechanisms of high-harmonic generation from crystals are described by
treating the electric field of a laser as a quasi-static strong field. Under
the quasi-static electric field, electrons in periodic potentials form dressed
states, known as Wannier-Stark states. The energy differences between the
dressed states determine the frequencies of the radiation. The radiation yield
is determined by the magnitudes of the inter-band and intra-band current matrix
elements between the dressed states. The generation of attosecond pulses from
solids is predicted. Ramifications for strong-field physics are discussed.
