The choroid is a multifunctional structure in the eye, primarily serving as a vascular supply for the outer retina. It contains large membrane-lined lacunae that function in lymphatic drainage and can change volume dramatically, affecting choroidal thickness. The choroid also contains non-vascular smooth muscle cells that may thin the choroid, counteracting changes in lacunae volume. Intrinsic choroidal neurons, present behind the central retina, may control these muscles and modulate choroidal blood flow, receiving sympathetic, parasympathetic, and nitrenergic innervation. The choroid's vascular functions include supplying oxygen and nutrients to the retina, regulating temperature, and modulating intraocular pressure. Additionally, it contains secretory cells that may influence vascularization and scleral growth. The dramatic changes in choroidal thickness allow the retina to move forward and back, adjusting the photoreceptors to the focal plane. These changes are correlated with scleral growth, suggesting a role in eye growth homeostasis and the etiology of myopia and hyperopia. The choroid's structure includes multiple layers, with the choriocapillaris being a highly permeable network of capillaries. The choroid is innervated by both sympathetic and parasympathetic systems, which regulate blood flow and muscle contraction. Intrinsic choroidal neurons and non-vascular smooth muscle cells are involved in adjusting choroidal thickness in response to retinal defocus. The choroid also contains fluid-filled lacunae, which may function as lymphatic structures, and its blood flow plays a role in thermoregulation of the retina. The choroid is involved in age-related macular degeneration, where changes in Bruch's membrane and choriocapillaris thickness contribute to retinal degeneration. The choroid's thickness can be modulated by refractive state and circadian rhythms, with changes in choroidal thickness linked to refractive adjustment and eye growth. The mechanisms underlying these changes include alterations in osmotically active molecule synthesis, vascular permeability, fluid flux from the anterior chamber, and movement of fluid across the RPE. These processes collectively contribute to the dynamic regulation of choroidal thickness and its role in maintaining retinal function and eye homeostasis.The choroid is a multifunctional structure in the eye, primarily serving as a vascular supply for the outer retina. It contains large membrane-lined lacunae that function in lymphatic drainage and can change volume dramatically, affecting choroidal thickness. The choroid also contains non-vascular smooth muscle cells that may thin the choroid, counteracting changes in lacunae volume. Intrinsic choroidal neurons, present behind the central retina, may control these muscles and modulate choroidal blood flow, receiving sympathetic, parasympathetic, and nitrenergic innervation. The choroid's vascular functions include supplying oxygen and nutrients to the retina, regulating temperature, and modulating intraocular pressure. Additionally, it contains secretory cells that may influence vascularization and scleral growth. The dramatic changes in choroidal thickness allow the retina to move forward and back, adjusting the photoreceptors to the focal plane. These changes are correlated with scleral growth, suggesting a role in eye growth homeostasis and the etiology of myopia and hyperopia. The choroid's structure includes multiple layers, with the choriocapillaris being a highly permeable network of capillaries. The choroid is innervated by both sympathetic and parasympathetic systems, which regulate blood flow and muscle contraction. Intrinsic choroidal neurons and non-vascular smooth muscle cells are involved in adjusting choroidal thickness in response to retinal defocus. The choroid also contains fluid-filled lacunae, which may function as lymphatic structures, and its blood flow plays a role in thermoregulation of the retina. The choroid is involved in age-related macular degeneration, where changes in Bruch's membrane and choriocapillaris thickness contribute to retinal degeneration. The choroid's thickness can be modulated by refractive state and circadian rhythms, with changes in choroidal thickness linked to refractive adjustment and eye growth. The mechanisms underlying these changes include alterations in osmotically active molecule synthesis, vascular permeability, fluid flux from the anterior chamber, and movement of fluid across the RPE. These processes collectively contribute to the dynamic regulation of choroidal thickness and its role in maintaining retinal function and eye homeostasis.