This review provides a comprehensive overview of research on gaze behavior and its effects on observers, focusing on the mechanisms of joint attention in infants, adults, and clinical populations. The gaze-cueing paradigm, which investigates how gaze direction shifts visual attention, has significantly contributed to understanding social cognition and attention. The "language of the eyes" conveys complex mental states, and gaze direction can automatically shift attention to the same object others are looking at. This phenomenon has been studied in infants for decades and is now being explored in adults as well. Visual attention, a classic area of cognitive research, has been reinvigorated with the use of ecologically valid stimuli, such as eyes. Cognitive spatial cueing paradigms have also benefited joint attention research. The integration of these domains has led to one of the most interesting fields of research in social cognition. The perception of direct and averted gaze has been extensively studied using brain imaging and electrophysiology in humans and nonhuman primates. The superior temporal sulcus (STS) is a central component of the neural system for social perception, responding to movements of the eyes, mouth, and body. The STS is also activated by static images of different postures of the face and body. Cells in the macaque STS are sensitive to different orientations of another's head and eyes. The STS is part of a wider network for social perception that includes processing of face identity. The STS is also heavily connected with the parietal cortex, which is implicated in orienting of attention. Information about eye-gaze direction could project to spatial attention systems to initiate orienting of attention in the corresponding direction, as in joint attention. Accurate gaze perception is achieved through a combination of luminance and geometry. Higher level factors also influence where we think someone is looking. The perception of gaze direction can be influenced by its face context. Eye contact has profound effects on the receiver, and can be perceived as an aggressive approach signal. Eye contact also acts as a signal of attraction between people. The ability to discriminate between direct and averted gaze is found across different species, and may have evolved because direct gaze can signal that a predator is attending. In clinical populations, such as children with autism, gaze perception and joint attention are impaired. Children with autism often avoid eye contact and have difficulty detecting gaze direction. They also have trouble attributing emotions to people based on the eye region. The development of gaze perception in infants may be due to the use of simple systems by higher level systems dedicated to the establishment of a sophisticated picture of other people's overt behavior, future intentions, and mental states. Gaze perception is crucial for joint attention and social cognition. Impediments to the normal development of these mechanisms can have profound implications for social cognition. Primates, including chimpanzees, show gaze following and some aspects of joint attention, but cannot use such cues to solve simple object-choice problems. Tamarind monkeys, squirrel monkeysThis review provides a comprehensive overview of research on gaze behavior and its effects on observers, focusing on the mechanisms of joint attention in infants, adults, and clinical populations. The gaze-cueing paradigm, which investigates how gaze direction shifts visual attention, has significantly contributed to understanding social cognition and attention. The "language of the eyes" conveys complex mental states, and gaze direction can automatically shift attention to the same object others are looking at. This phenomenon has been studied in infants for decades and is now being explored in adults as well. Visual attention, a classic area of cognitive research, has been reinvigorated with the use of ecologically valid stimuli, such as eyes. Cognitive spatial cueing paradigms have also benefited joint attention research. The integration of these domains has led to one of the most interesting fields of research in social cognition. The perception of direct and averted gaze has been extensively studied using brain imaging and electrophysiology in humans and nonhuman primates. The superior temporal sulcus (STS) is a central component of the neural system for social perception, responding to movements of the eyes, mouth, and body. The STS is also activated by static images of different postures of the face and body. Cells in the macaque STS are sensitive to different orientations of another's head and eyes. The STS is part of a wider network for social perception that includes processing of face identity. The STS is also heavily connected with the parietal cortex, which is implicated in orienting of attention. Information about eye-gaze direction could project to spatial attention systems to initiate orienting of attention in the corresponding direction, as in joint attention. Accurate gaze perception is achieved through a combination of luminance and geometry. Higher level factors also influence where we think someone is looking. The perception of gaze direction can be influenced by its face context. Eye contact has profound effects on the receiver, and can be perceived as an aggressive approach signal. Eye contact also acts as a signal of attraction between people. The ability to discriminate between direct and averted gaze is found across different species, and may have evolved because direct gaze can signal that a predator is attending. In clinical populations, such as children with autism, gaze perception and joint attention are impaired. Children with autism often avoid eye contact and have difficulty detecting gaze direction. They also have trouble attributing emotions to people based on the eye region. The development of gaze perception in infants may be due to the use of simple systems by higher level systems dedicated to the establishment of a sophisticated picture of other people's overt behavior, future intentions, and mental states. Gaze perception is crucial for joint attention and social cognition. Impediments to the normal development of these mechanisms can have profound implications for social cognition. Primates, including chimpanzees, show gaze following and some aspects of joint attention, but cannot use such cues to solve simple object-choice problems. Tamarind monkeys, squirrel monkeys