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A general expression is derived for the maximum angular accuracy of tracking a radio star by lobe comparison (or monopulse). This angular accuracy depends on the input signal-to-noise ratio, the wavelength, the time-bandwidth product of signal integration, and the effective length of the antenna aperture. The maximum angular accuracy can be obtained, approximately, by performing a simple correlation of odd and even components of the antenna output. Angular accuracy formulas for simple antenna dishes or for interferometers appear as special cases of the general result. The Appendix discusses the interferometer technique in more detail, and the angular accuracy for the data processing technique used by M. Ryle is compared with that obtained from the optimum processing.