Violating the Uncertainty Principle: A Review

We review the difference between two uncertainties: the \textit{preparation }uncertainty, which involves states and observables and the \textit{measurement} uncertainty, which also incorporates the evolution implied in a physical measurement process. We explore the proposed concepts of \textit{error} and \textit{disturbance}, which enter the measurement uncertainty and review recent experiments showing evidence for a violation of this \textit{measurement} uncertainty. We examine selected proposed reformulations of the measurement uncertainty relation and evaluate how closely they approach the experimental data.