  Molecular dynamics (MD) simulations allow investigating the structural
dynamics of biomolecular systems with unrivaled time and space resolution.
However, in order to compensate for the inaccuracies of the utilized empirical
force fields, it is becoming common to integrate MD simulations with
experimental data obtained from ensemble measurements. We here review the
approaches that can be used to combine MD and experiment under the guidance of
the maximum entropy principle. We mostly focus on methods based on Lagrangian
multipliers, either implemented as reweighting of existing simulations or
through an on-the-fly optimization. We discuss how errors in the experimental
data can be modeled and accounted for. Finally, we use simple model systems to
illustrate the typical difficulties arising when applying these methods.
