Objective: To examine the temperament and sensory processing factors related to the social participation of 3 year olds with developmental delays. Method: This is cross-sectional, descriptive study. Parent responses on questionnaires for temperament and sensory processing were correlated with measures of social participation. Predictors for social participation were explored. Results: The temperament factor of Effortful Control was found to be moderately related to social participation. The total sensory score was significantly and strongly related to social participation. The sensory scale score of Body Awareness had a correlation of .535 and Planning Ideas .593 with social participation. The total sensory score was the strongest predictor of social participation. Conclusion: Young children wit developmental delays have temperament and sensory processing vulnerabilities that place them at high risk for difficulties in social skill development and their social participation at home, school, and community. Pediatric therapists can provide interventions to support the foundations of social participation to include sensory integration therapy, cognitive approaches to learning social skills, and parental behavioral management to modify difficult temperament responses.