Using observations obtained by acoustic Doppler profilers and coastal water level recorders, we describe the vertical and horizontal structure of the currents and sea level due to the principal tidal constituents in Long Island Sound, a shallow estuary in southern New England. As expected, the observations reveal that M2 is the dominant constituent in both sea surface and velocity at all depths and sites. We also find evidence that the vertical structure of the M2 tidal current ellipse parameters vary with the seasonal evolution of vertical stratification at some sites. By comparing our estimates of the vertical structure of the M2 amplitudes to model predictions, we demonstrate that both uniform and vertically variable, time invariant eddy viscosities are not consistent with our measurements in the Sound. The current records from the western Sound contain significant overtides at the M4 and M6 frequencies with amplitudes and phases that are independent of depth. Though the M4 amplitude decreases to the west in proportion to M2, the M6 amplifies. Since the dynamics that generate overtides also produce tidal residuals, this provides a sensitive diagnostic of the performances of numerical circulation models. We demonstrate that the observed along-Sound structure of the amplitude of the M4 and M6 overtides is only qualitatively consistent with the predictions of a nonlinear, laterally averaged layer model forced by a mean flow and sea level at the boundaries. Since neither the vertical structure of the principal tidal constituent nor the pattern of horizontal variation of the largest overtides can be explained using well established models, we conclude that they are fundamentally inadequate and should no longer be used for more than a basic qualitative understanding, and even then should be used with caution. We provide comprehensive tables of the tidal current parameters to facilitate the critical evaluation of future models of the circulation in the Sound.