Two experiments investigated learning outcomes and comprehension processes when students learned about the heart and circulatory system using (a) text only, (b) text with simplified diagrams designed to highlight important structural relations, or (c) text with more detailed diagrams reflecting a more accurate representation. Experiment 1 found that both types of diagrams supported mental model development, but simplified diagrams best supported factual learning. Experiment 2 replicated learning effects from Experiment 1 and tested the influence of diagrams on novices' comprehension processes. Protocol analyses indicated that both types of diagrams supported inference generation and reduced comprehension errors, but simplified diagrams most strongly supported information integration during learning. Visual representations appear to be most effective when they are designed to support the cognitive processes necessary for deep comprehension.