Everyone knows that measures of lower-level word processes, higher-level processes, and working memory predict comprehension ability. Further, a number of comprehension theories suggest how some of these sources might relate. Yet, many of these relationships have gone untested and so it is unclear whether one or all of the sources make separate and important contributions to comprehension performance. The present study focuses on this issue. The results replicated a number of previous findings; however they also revealed that although the three sources made unique contributions to comprehension performance, lower-level and higher-level processes were the most important contributors.