The ability of an externally-pressurized slider to follow faithfully the runout, or waviness of a rotating disk, or drum are investigated. The response and the stability of the slider are considered in terms of small displacements from the equilibrium gap width. The first part of the analysis treats the case of an incompressible lubricant. The dynamic Reynolds equation is integrated with respect to the space coordinate and the relative displacement of the slider is described by a nonlinear, second-order differential equation. Peturbation solutions are obtained, which permit successive approximations of small deviations from the equilibrium gap width. The second part of the analysis treats the case of a gaseous lubricant. A quasi-static variation of the pressure field is assumed and the problem is stated in terms of lumped parameters. The continuity equation and the equation of motion are linearized, yielding a third-order differential equation for small displacements of the slider from the equilibrium gap width. Results are discussed, with particular reference to the effect of the squeeze number, σ, on the response of the slider. © 1964 by ASME.