Although a considerable amount of evidence has been amassed regarding the contributions of lower-level word processes, higher-level processes, and working memory to reading comprehension, little is known about the relationships among these sources of individual differences or their relative contributions to reading comprehension performance. This study addresses these shortcomings by using structural equation modeling. The principal structural equation model tested in this study--called the cognitive components-resource model of reading comprehension--proposes a set of specific relationships among lower-level word processing, higher-level processes, and working memory. This model is then systematically compared with a series of other models that propose alternative relationships among these three sources of individual differences. The results show that, although working memory influences higher-level processes, speed of lower-level word processing exerts little to no influence on higher-level processes or working memory. The results also show that a variant of the cognitive components-resource model of reading comprehension accounts for 62% of the variance in reading comprehension performance. Taken as a whole, the present study informs theories of reading comprehension by proposing relationships among important sources of individual differences. It also provides a foundation for future research seeking to test and compare theories of reading comprehension and other sources of individual differences. (Contains 7 notes, 3 tables, and 7 figures.)