  We discuss recent progress towards developing accurate initial state
descriptions for heavy ion collisions focusing on weak coupling based
approaches, that enable one to constrain the high-energy structure of nuclei
from deep inelastic scattering or proton-nucleus collisions. We review recent
developments to determine the event-by-event fluctuating nuclear geometry, to
describe gluon saturation phenomena at next-to-leading order accuracy, and to
include longitudinal dynamics to the initial state descriptions.
