The gain of a hearing aid was measured on each of eight subjects using a variety of functional (subjective) and insertion (objective) gain methods. Each measurement was repeated on subsequent days and the hearing aid gain was also determined in several couplers and on KEMAR. The multiple measurements enabled an accurate estimate of the “true” gain of the aid on each subject to be determined. The real ear gain measured by each of the individual methods was then compared with this true gain estimate and the relative accuracy of each method was quantified. Of the clinically feasible methods, probe microphone measurement of insertion gain proved to be the most accurate. Few significant differences were found between functional gain and insertion gain. Estimation of real ear gain on the basis of coupler gain appeared to be a reasonable, though not recommended approach.