The present talk aims at explaining the physical phenomena governing the interaction of light, paper and ink halftones and at presenting classical spectral reflectance prediction models. Physical phenomena comprise surface reflections and refractions at the air-paper interface, the propagation of light within the paper, internal reflections at the paper-air interface, ink spreading and trapping. We review classical reflectance prediction models such as the spectral Neugebauer model, the Yule-Nielsen modified spectral Neugebauer model, and the multiple reflection Clapper-Yule model. We then discuss the phenomena of dot gain and ink spreading and show how to take them into account. Finally, we give an overview of recent progress in the field and point to yet unsolved problems.