The perceived value of paper products depends not only upon their performance but also upon their visual appeal. The optical properties of paper, including whiteness, brightness, opacity, and gloss, affect its visual perception and appeal. From a practical point of view, it is important to quantify these optical properties by means of reliable and repeatable measurement methods, and furthermore, to relate these measured values to the structure of paper and characteristics of its constituents. This would allow papermakers to design new products with improved quality and reduced cost. In recent years, signiﬁcant progress has been made in terms of the fundamental understanding of light-paper interaction and its effect on paper’s appearance. The introduction of digital imaging technology has led to the emergence of a new category of optical testing methods and has provided fresh insights into the relationship between paper’s structure and its optical properties. These developments were complemented by advances in the theoretical treatment of light propagation in paper. In particular, wave scattering theories in random media are ﬁnding increasing applicability in gaining a better understanding of the optical properties of paper. In this document, a review of these advancements is presented.