An accurate understanding of the initial state of heavy-ion collisions is crucial to the interpretation of the corresponding measurements. The initial state geometry, including the spatial distribution of nucleons and their fluctuations, is most often modeled using the Glauber model, which can be tested using two-particle correlations or electroweak boson measurements. The parton distribution functions, encoding information about their longitudinal momentum, are still poorly constrained, though LHC data provides new stringent inputs. Future impact from planned experimental upgrades and new facilities are also discussed.