Research with school-age readers suggests that the contributions of reading and language skills vary across reading comprehension assessments and proficiency levels. With a sample of 168 struggling adult readers, we estimated the explanatory effects of decoding, oral vocabulary, listening comprehension, fluency, background knowledge, and inferencing across three reading comprehension tests and across low, average, and high levels of performance. OLS regression models accounted for 66% of the variance in WJ Passage Comprehension scores with all competencies except listening comprehension as significant predictors; 43% of the variance in RAPID Reading Comprehension scores with decoding and listening comprehension as significant predictors; and 31% of the variance in RISE Reading Comprehension scores with decoding as a significant predictor. Quantile regression models and between-quantile slope comparisons showed that the effects of some predictors on reading comprehension varied across performance levels on one or more tests. Implications for instruction, assessment, and future research are discussed. [For the corresponding grantee submission, see ED610419.]