The goal of this study was to examine the role of domain and system knowledge on learner performance in reading and information search in hypermedia. Previous studies have shown that prior knowledge is an important individual factor for effective hypermedia use. However, current research lacks a full understanding of how these two aspects of prior knowledge affect learner performance in hypermedia. The experiment was carried out as 2x3 factorial design with two factors: learn goal (reading vs. searching) and prior knowledge (no prior knowledge, domain knowledge, system knowledge). The results show that domain knowledge improved knowledge acquisition, but had only a small effect on learners' search performance, whereas system knowledge improved search performance, but had only little effect on knowledge acquisition. Navigation performance data revealed that participants with domain or system knowledge navigated in a more goal oriented way than participants without prior knowledge. (Contains 2 tables and 2 figures.)