眼（亦称眼睛、目、招子）是视觉的器官，可以感知光线，转换为神经中电化学的脉冲。比较复杂的眼睛是一个光学系统，可以收集周遭环境的光线，借由虹膜调整进入眼睛的强度，利用可调整的晶状体来聚焦，投射到对光敏感的视网膜产生影像，将影像转换为电的信号，透过视神经传递到大脑的视觉系统及其他部份。眼睛依其辨色能力可以分为十种不同的种类，有96%的动物其眼睛都是复杂的光学系统。其中软体动物、脊索动物及节肢动物的眼睛有成像的功能。

微生物的「眼睛」构造最简单，只侦测环境的暗或是亮，这对于昼夜节律的牵引有关。若是更复杂的眼睛，视网膜上的感光神经节细胞沿着视网膜下视丘路径发送信号到视叉上核来影响影响生理调节，也送到顶盖前核控制瞳孔光反射。

Eyes are the organs of vision. They detect light and convert it into electro-chemical impulses in neurons. In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.