The successful application to many problems of data derived from satellite radiometers requires a clear determination of the accuracies and stability of the measurements. Since access to satellite radiometers for direct testing is not possible after launch, a range of approaches has to be employed to assess the performance of the instrument. These approaches involve using information derived from the instruments themselves and making comparisons between top-of-atmosphere radiance measurements from several satellites operating at the same time. Using sea-surface temperature (SST) retrievals as an example, further approaches are described that involve comparisons between SST fields derived from different satellites, and comparisons with independent measurements from sensors at or below the sea surface. All have strengths and weaknesses, but a combination of several approaches can be used to build confidence in the useful accuracy of the satellite instruments and the derived SSTs.