Compared to other children, children with Autism Spectrum Disorder (ASD) are known to engage less in joint attention - the sharing of attention between two individuals toward a common object or event. Joint attention behaviors - for example gaze following, alternating gaze, and pointing - play an important role in early development, as they provide a foundation for learning and social interaction. Study I and Study II focused on infant siblings of children with ASD. These infants, often termed high risk (HR) infants, have an increased probability of receiving a later ASD diagnosis. Studying them therefore allows for the detection of early signs of ASD. Live eye tracking was used to investigate different joint attention behaviors at 10 months of age. Study I showed that omitting the head movement that usually accompany experimenters’ eye gaze shifts in similar designs reduced gaze following performance in the HR group, but not in a group of infants at low risk (LR) for ASD. HR infants may thus be less sensitive to eye information, or may need more salient cues in order to follow gaze optimally. Study II focused on the infants’ tendency to initiate joint attention by alternating their gaze between a person and an event. LR infants engaged more in alternating gaze than HR infants, and less alternating gaze in infancy was associated with more ASD symptoms at 18 months. This relation remained when controlling for visual disengagement and general social interest in infancy. Study III explored the role of joint attention later in development, by investigating the microstructure of the looking behaviors of autistic and typically developing children (~6 years old). The results indicated that seeing somebody look at an object influenced the processing of that object less in autistic children than in the typically developing controls. Both groups followed gaze effectively, suggesting that differences in joint attention at this age may be subtle, but detectable with eye tracking technology. Together, the studies contribute to our understanding of the role that joint attention atypicalities play both in the early development of infants at risk for ASD, and later in the development of children with a confirmed diagnosis.