We study the effects of both AC and DC fields in the quasienergy band structure of cold atoms in optical lattices. The effects studied include Rabi oscillations between Bloch bands and dynamical band collapse in AC fields, which have already been observed experimentally. We also present some new results regarding the location of Wannier-Stark ladders in DC fields and the band structure of fractional Wannier-Stark ladders in DC/AC fields.