Photosynthesis is one of nature’s paramount phenomena, occurring in chlorophyll-containing organisms, highlighting leaves as the primary photosynthetic organ. Leaves are also the main host plant organ attacked by gall-inducing organisms, whose establishment and subsequent development lead to gall development, imposing a biotic stress and triggering tissue redifferentiation. That process changes the photosynthesis profile, which may negatively impact the host’s primary productivity. The current chapter examines the effects of gall induction and development on chlorophyllous tissues, photosynthesis rates, and pigment concentration. The main processes involved in gall development, tissue hyperplasia and cell hypertrophy, may reduce photosynthetic pigment content or promote its compartmentalization at the outermost tissue layers of the galls. Furthermore, the quantum yield of the PSII decreases in gall tissues but seems to develop an important role in tissue homeostasis by producing oxygen and consuming carbon dioxide, avoiding hypoxia and hypercarbia. Although the adaptive role of photosynthesis in green galls has not been extensively tested, evidence indicates its crucial role in tissue homeostasis and the maintenance of gall metabolism.