Changes observed in the foliage of trees killed by bark beetles are usually described in terms of stages that have been related to a specific timeframe. The “green attack” stage is the period of time, typically one year long, in which the needles of newly colonized trees remain essentially green before turning gradually red. A review of the existing literature is presented in this report as an attempt, based on existing knowledge, to evaluate the possibility of detecting, with remote sensing, infested areas of forests in the “green attack” stage of infestation. The emphasis is on the expected biophysical changes of the trees and their foliage. Other important aspects, such as sensor characteristics and observation strategies, have not been considered here. The process of tree colonization by the bark beetle is presented to highlight the most important aspects of the insect–host interaction. The controversy about the role of the fungi in killing the tree is then reviewed. Attacked trees will likely experience a progressively severe water stress caused by xylem disruption. The expected response of a plant under moisture deficit and the possibility that the tree may offset the symptoms is discussed. Finally, a brief review of works on remote sensing detection of vegetation water content, vegetation stress, and foliar nitrogen is presented to demonstrate the capability of this technique to retrieve information on bark beetle outbreaks.