Measurements have been made of the onset of instability in rotating Couette flow in order to check some recent numerical results. The case of a very wide gap, a medium gap, and a very narrow clearance ratio have been studied. The accuracy of the results is about ± 2%. The present data and other values available in the literature are compared with the computations of different investigators. With a few exceptions the agreement is found to be excellent. End effects are described and their relative importance is discussed. Methods of scaling the results are evaluated. A simple formula is presented for calculating the onset value when only the inner cylinder rotates.