  This lecture gives an overview of the impacts on linear machine optics of
machine imperfections due to incorrect field settings and misalignments. The
effects of imperfections in dipole, quadrupole, and sextupole magnets are
presented, along with beam observables and correction techniques that may be
used to restore the nominal machine parameters. The main concepts of orbit
correction are discussed in detail, because the principles underlying those
techniques can be used for other corrections.
