The optical quality of paper is predominantly judged by the customer on the basis of appearance and/or on the basis of standardised measurements such as gloss, whiteness, brightness, and opacity that try to quantify appearance sensations. The common Kubelka-Munk (KM) model approach to interpret diffuse reflectance factor measurements of paper has worked as a guide for the optical design of paper for many applications. However, because the KM model neither describes the structure of the scattering medium nor the optical response in much detail, both the interpretation and the predictability of diffuse reflectance factor measurements based on the KM model become uncertain. In addition, when the measured values of a paper's optical quality, such as whiteness, opacity, and gloss, do not accurately describe the paper quality that is visually perceived by the customer in the value chain, there is a need for better quality measures. This thesis takes a serious look at the angle dependent light scattering from paper as a critical issue when attempting to correlate measured optical properties to appearance features. It is done in order to pave the way for a better understanding of the optical response of paper, which in turn is a requirement for design and development of paper products with dedicated optical performance. In addition the possibilities of extending the validity and the predictability of the necessary modelling tools are investigated. We have found that the diffuse 3-D reflectance distribution from fibre containing samples show a forward scattering lobe that is distinctly different from the much more Lambertian distribution from coated samples. (...).