Recently, several studies have claimed that soon after their first birthday infants understand others' false beliefs. However, some have questioned these findings based on criticisms of the looking-time paradigms used. Here we report a new paradigm to test false belief understanding in infants using a more active behavioral response: helping. Specifically, the task was for infants to help an adult achieve his goal - but to determine that goal infants had to take into account what the adult believed (i.e., whether or not he falsely believed there was a toy inside a box). Results showed that by 18 months of age infants successfully took into account the adult's belief in the process of attempting to determine his goal. Results for 16-month-olds were in the same direction but less clear. These results represent by far the youngest age of false belief understanding in a task with an active behavioral measure. (Contains 4 figures.)