The authors investigated the effects of domain knowledge on online reading among younger and older adults. Individuals were randomly assigned to either a domain-relevant (i.e., high-knowledge) or domain-irrelevant (i.e., low-knowledge) training condition. Two days later, participants read target passages on a computer that drew on information presented in the high-knowledge training session. For both age groups, knowledge improved comprehension and recall and facilitated the processing of topic shifts during reading. In addition, domain knowledge had differential effects on processing across the 2 age groups. Among older (but not younger) readers, domain knowledge increased the time allocated to organization and integration processes (wrap-up) and increased the frequency of knowledge-based inferences during recall. These results suggest that among older readers, domain knowledge engenders an investment of processing resources during reading, which is used to create a more elaborated representation of the situation model.