This paper surveys the problem of modelling the dynamics of large ice sheets. A simplified model for two-dimensional plane ice sheets is derived, and both isothermal and non-isothermal cases are considered. The model is not uniformly asymptotically valid at a divide or at a margin, and we suggest local (isothermal) analyses which give order of magnitude estimates for divide curvature and margin slope. We also give a uniformly valid description for small perturbations to an isothermal ice sheet, which decay diffusively. For the more interesting non-isothermal case, we are able to provide explicit approximate solutions for the surface profile, based on Lliboutry's heuristic boundary layer analysis, and give an approximate description of the temperature field.