We examined young readers' comprehension as a function of text genre (narrative, science), text cohesion (high, low), and readers' abilities (reading decoding skills and world knowledge). The overarching purpose of this study was to contribute to our understanding of the "fourth grade slump". Children in grade 4 read four texts, including one high and one low cohesion text from each genre. Comprehension of each text was assessed with 12 multiple-choice questions and free and cued recall. Comprehension was enhanced by increased knowledge: high knowledge readers showed better comprehension than low knowledge readers and narratives were comprehended better than science texts. Interactions between readers' knowledge levels and text characteristics indicated that the children showed larger effects of knowledge for science than for narrative texts, and those with more knowledge better understood the low cohesion, narrative texts, showing a reverse cohesion effect. Decoding skill benefited comprehension, but effects of text genre and cohesion depended less on decoding skill than prior knowledge. Overall, the study indicates that the fourth grade slump is at least partially attributable to the emergence of complex dependencies between the nature of the text and the reader's prior knowledge. The results also suggested that simply adding cohesion cues, and not explanatory information, is not likely to be sufficient for young readers as an approach to improving comprehension of challenging texts.