From a surprisingly young age, children are capable of a level of sophisticated social understanding and interaction for which they are rarely given credit. Indeed, beginning as early as 9 months, children are gaining the skills needed to become active members of the unique culture and society into which they are born. This article explores the developmental trajectory of shared intentionality--collaborative activity in which participants share psychological states with one another--which starts infants on the road to full participation in human society in the second year of life. A better understanding of infants' capabilities in sharing intentions with others will lead to richer and more appropriate interactions with them.