The BESSY II undulators produce brilliant photon beams which are limited only by the electron beam emittance and energy spread. The predicted spectral performance including polarization fits well with measurements. Since the devices are integral parts of the machine the interaction with the electron beam has to be minimized. Sophisticated strategies have been developed for passive and active compensation schemes. Advanced software packages have been developed at the HZB to describe the beam optic effects as well as the spectral undulator performance and its propagation along the beamline. The existing know-how in the field of undulator construction and fabrication is the basis for new challenging undulator projects for storage ring upgrades or future light source.