The detection of an electromagnetic counterpart to the gravitational-wave source GW170817 marked year zero of the multi-messenger gravitational-wave era. This event was generated by the merger of two neutron stars and gave rise to an electromagnetic transient, dubbed a kilonova, which was intensively monitored with all the main ground-based and space-borne facilities. The general agreement between existing models and data is remarkable; however, critical parameters like the inclination of the system and the distribution of the ejecta components are still uncertain despite their being crucial to e.g. calculate kilonova rates and estimate the Hubble constant. In this talk, I will present POSSIS, a Monte Carlo radiative transfer code that is well- suited to study multi-dimensional kilonova models. In particular, I will show how the predicted polarization signatures and viewing-angle dependent spectra and light curves can be used to place constraints on models, help interpret data and drive future kilonova observations.