Proportional thinking, which requires understanding fractions, ratios, and proportions, is an area of mathematics that is cognitively challenging for many children and adolescents (Fujimura, 2001; Lamon, 2007; Lobato, Ellis, Charles, & Zbiek, 2010; National Mathematics Advisory Panel [NMAP], 2008) and "transcends topical barriers in adult life" (Ahl, Moore, & Dixon, 1992, p. 81). Problems associated with the failure to understand proportional concepts and operations, and difficulty distinguishing the multiplicative relationship between rational quantities from earlier learned additive arithmetic concepts, are key obstacles to progress in more advanced mathematics, including algebra (Boyer, Levine, & Huttenlocher, 2008; NMAP, 2008). This study aimed to develop greater understanding of why proportional thinking continues to be an area of great difficulty for students. In particular, it seems clear that solving proportion problems, whether they are presented in symbolic form or as word problems, require the coordination of a variety of domain-specific knowledge (including conceptual and procedural knowledge). Unfortunately, little is known about how these types of domain-specific knowledge interact and contribute to the development of proportional word problem solving. Identifying which of these variables is important in promoting the development of proportional thinking (and conversely which are implicated in those who have difficulty with proportional word problem solving) is critical to inform future interventions for improving students' performance in this area. Prior research is summarized on domain-specific knowledge (i.e., fractions, proportions) as it provides the basis for the hypotheses that certain variables are important for proportional word problem solving. The following research question directed this study: Does domain-specific knowledge (conceptual and procedural fraction knowledge, conceptual and procedural proportion knowledge) predict proportional word problem solving? All 429 seventh-grade students from 17 classrooms at three U.S. middle schools in two suburban Midwest school districts participated in the study. Results indicated that conceptual fraction knowledge, procedural fraction knowledge, conceptual proportion knowledge, and procedural proportion knowledge were significant predictors of proportional word problem solving performance. The results support the notion that both conceptual fraction and proportion knowledge and procedural fraction and proportion knowledge play a major role in understanding individual differences in proportional word problem solving performance to inform interventions. Tables and figures are appended.