Abstract: Prior knowledge of the content of a passage should reduce the effort required to encode the passage, and facilitate its recall. This paper presents the results of two experiments on the effects of prior knowledge upon comprehension of simple technical prose. The basic procedure was to collect ratings of the amount of prior knowledge for individual passage sentences, and then determine whether those familiarity ratings predict reading 'study) time and recall for the sentences. Three encoding task conditions were used in the same basic procedure: a self-paced study task, a force-paced study task, and an incidental learning task. A cued recall test followed each condition. In the self-paced task, readers studied unfamiliar material longer than familiar material, but recalled at the same level regardless of familiarity. In contrast, familiarity did predict recall in the force-paced and incidential tasks. Unknown proposititons were recalled less often than known propositions; the recall of unknown propositions was especially low in the incidental task. An explanation for this pattern of results is based on the idea that increased study time is required for unfamiliar material due to the additional processing required to encode an unknown proposition and to elaborate less familiar propositions.