Although there is evidence that 18 month-olds understand human behavior as intentional, little is known about the developmental antecedents of this ability. In the first part of the paper empirical findings on intention understanding in the second year of life are reviewed and discussed. The second part of the paper reports a longitudinal study investigating children's intention understanding. A total of 40 children (19 girls) were observed at 12 and 15 months of age. At both ages, children were administered the "Demonstration of Intention" condition in the Re-enactment paradigm devised by Meltzoff (1995). The study had three goals. The first goal was to replicate earlier findings on intention understanding in 12- and 15-month-olds longitudinally assessed. The second goal was to analyse which actions children perform when not producing the target act. The third goal was to explore whether there were individual differences in intentional understanding. Results confirmed previous findings, i.e. children's tendency to produce the target act was higher at an older age. It was found that individual children with a tendency to perform high intention understanding at 12 months tended to remain high at 15 months, thus raising the possibility of stable individual differences in intention understanding.