  Excited state contributions represent a formidable challenge for hadron
structure calculations in lattice QCD. For physical systems that exhibit an
exponential signal-to-noise problem they often hinder the extraction of ground
state matrix elements, introducing a major source of systematic error in
lattice calculations of such quantities. The development of methods to treat
the contribution of excited states and the current status of related lattice
studies are reviewed with focus on nucleon structure calculations that are
notoriously affected by excited state contamination.
