We look at the influence of surface topography on ice nucleation. For nucleation from supercooled water, this is shown to be unimportant for several surface materials, but for nucleation from a supersaturated vapour topography is seen to be extremely important. Acute wedge features on cleaved mica provide ideal model nucleation sites, showing the importance of acute geometries to nucleation. Experiments with organic analogues to ice reveal the mechanism: an acute geometry promotes growth of a confined phase in under saturated conditions, which grows into a bulk phase in supersaturated conditions.