How accurately can a single molecule be localized when imaged through an optical microscope?

We present a simple analytical expression for the fundamental limit to the accuracy with which the location of a single molecule can be determined that is imaged through an optical microscope. This expression depends on the optical properties of the microscope and the photophysical properties of the single molecule. We also show how the fundamental limit is deteriorated by factors like pixelation of the detector and noise sources in the detection system. The present results gives an experimenter insight into what is achievable in an optical microscope and provide guidelines for experimental design.