This chapter presents the theoretical concepts necessary to link optical sensor data to forest structural attributes . Forest stand architecture is relatively complex and spatially variable, and many approaches can be used for its measurement: from traditional forest measurements in ground plots , to the statistical representation of forest attributes with the use of allometric relationships , and to the use of optical sensors. The forest parameters routinely measured are presented followed by the specific measurements that optical field instruments provide. Emphasis is placed on the optical sensors measuring light transmission through the forest canopy using the hemispherical view approach and, more specifically, on the use of hemispherical photographs. Strengths and weaknesses of forest canopy structural attributes measured based on Beer’s law , in particular from canopy gaps as seen from a hemispherical sensor or photograph, are presented. Finally, scaling issues are introduced from single trees to a stand and from stands to area mapping with satellite sensing.