We review general properties and capabilities of the instrumentation employed to diagnose cosmic magnetic fields using medium-size and large optical telescopes. During the last decade these spectropolarimeters and high-resolution spectrographs have been successfully used to detect and characterize magnetic fields in stars across the H-R diagram. A new generation of high-resolution spectropolarimeters will benefit from the large collecting area of the future E-ELT and currently operating 8-m class telescopes. We review plans to develop spectropolarimeters for these very large telescopes and outline a number of science cases where new spectropolarimetric instrumentation is expected to play a key role.
