Young children's social information processing (SIP) encompasses a series of steps by which they make sense of encounters with other persons; both cognitive and emotional aspects of SIP often predict adjustment in school settings. More attention is needed, however, to the development of preschoolers' SIP and its potential foundations. To this end, a new preschool SIP measure, the Challenging Situations Task (CST), was utilized; preschoolers' ( = 316) self-reported emotional and behavioral responses to hypothetical peer provocation situations on the CST were assessed longitudinally, along with aspects of their self-regulation and emotion knowledge. Age and developmental differences in CST responses were examined. Next, contributions of executive control and emotion knowledge to CST responses were analyzed. Age differences in emotion and behavior choices showed that younger preschoolers were more prone to choose happy responses, whereas older preschoolers chose more adaptive behavior responses. Both self-regulation and emotion knowledge were associated with emotion and behavior responses concurrently and across time. Implications of these findings and suggestions for further research are discussed.