Predictions of marine ice-sheet behaviour require models able to simulate grounding-linemigration. We present results of an intercomparison experiment for plan-view marine ice-sheet models.Verification is effected by comparison with approximate analytical solutions for flux across thegrounding line using simplified geometrical configurations (no lateral variations, no buttressing effectsfrom lateral drag). Perturbation experiments specifying spatial variation in basal sliding parameterspermitted the evolution of curved grounding lines, generating buttressing effects. The experimentsshowed regions of compression and extensional flow across the grounding line, thereby invalidating theboundary layer theory. Steady-state grounding-line positions were found to be dependent on the level ofphysical model approximation. Resolving grounding lines requires inclusion of membrane stresses, asufficiently small grid size (<500 m), or subgrid interpolation of the grounding line. The latter stillrequires nominal grid sizes of <5 km. For larger grid spacings, appropriate parameterizations for ice fluxmay be imposed at the grounding line, but the short-time transient behaviour is then incorrect anddifferent from models that do not incorporate grounding-line parameterizations. The numerical errorassociated with predicting grounding-line motion can be reduced significantly below the errorsassociated with parameter ignorance and uncertainties in future scenarios.