The soundness error of a PCP verifier is the probability (usually denoted ) that the verifier accepts an incorrect input. We are interested in the smallest possible values of for which the PCP theorem holds, and in particular whether the theorem holds when is an inverse polynomial function of the input length. We discuss the `sliding scale conjecture' of [BGLR93, LY94] and related questions. We then sketch some of the existing approaches and constructions of PCPs with sub-constant soundness error.