Supporting students in becoming effective problem solvers is a critical component of K-12 mathematics instruction. Unfortunately, little is known about the factors that are related to problem solving proficiency in middle school students. We report the results of a study that employed a hierarchical linear regression analysis to examine the factors that influenced the problem-solving scores in a sample of 213 sixth and seventh grade students. Results support theoretical predictions that metacognition, executive function, student beliefs, and content knowledge all relate to problem solving proficiency. [For the complete proceedings, see ED658295.]