Undulators for SASE FELs

In this contribution, the requirements on undulators for linac driven SASE FELs will be discussed. Differences between long and short wavelength SASE FELs will be worked out. The problematic influencing the choice of minimum gap and undulator peak field which are special for SASE FELs driven by multi GeV electron beams with sub picosecond pulses, peak currents of several kiloamperes will be pointed out. Special attention is given to the magnetic design of combined strong focusing undulators as are needed for VUV FELs. As an example results will be presented of the undulator for the VUV-FEL at the TESLA Test Facilty which has just been completed.