[en] Two different types of insertion devices - undulators and wigglers - are described and compared. Each provides a periodic magnetic field that alters polarity, but whereas the spectral distribution from a wiggler is continuous and wide, the radiation of an undulator has spectrally narrow and discrete peaks. The distinction is determined by the deflection parameter. The energy spread in undulator peaks is calculated. Insertion device magnets and gap are discussed. Undulator tunability from gap variation is considered on a 6-GeV lattice and on a 7-GeV lattice. Also discussed is the angular distribution of power and the polarization from various sources