Many factors make the implementation of a magnetic optics in a circular accelerator different from the nominal one: deviations from the magnet calibration curves and from the ideal magnetic lengths, displacements from the reference position and axis, unknown multipole components, and the like. This generally results in machine performances below expectations: low beam lifetime and dynamic aperture, poor injection efficiency, large emittances (in lepton machines) and limited luminosity (in colliders).

The stored beam may be used to infer such magnetic imperfections. This talk will review the beam-based techniques which are being used at the ESRF, some of them well established since decades, some other conceived only recently, and one still under development. Pros and cons of each scheme are discussed.