How do people plan ahead in sequential decision-makingtasks? In this article, we compare computational models of hu-man behavior in a challenging variant of tic-tac-toe, to inves-tigate the cognitive processes underlying sequential planning.We validate the most successful model by predicting choicesduring games, two-alternative forced choices and board evalu-ations. We then use this model to study individual skill differ-ences, the effects of time pressure and the nature of expertise.Our findings suggest that people perform less tree search un-der time pressure, and that players search more as they improveduring learning.