This study examined the influence of deliberate practice, defined as practice specifically aimed at learners' weak areas and only their weak areas, on 8th graders performance in geometry. A control group had a choice over practice problems and their sequencing. Experiment 1 indicated a disordinal practice schedule by knowledge interaction. Simple effects tests indicated that the interaction was primarily caused by less knowledgeable learners benefiting more from a self-selected practice schedule than deliberate practice. Two subsequent experiments explored the cognitive mechanisms behind this effect by using learners with different levels of prior knowledge. Whereas the relatively more knowledgeable learners in Experiment 2 benefited by concentrating only on their weak areas during practice, the less knowledgeable learners in Experiment 3 improved their skills when they practiced on problem sets combining some of their weak and some of their strong areas or by concentrating on only a limited number of weak areas for a given problem area. These findings have important implications for the design of curriculum materials and implementation of deliberate practice techniques in secondary classrooms. Prior to attaining a sufficient level of familiarity with the subject matter, learners should be encouraged to continue practicing in areas in which they have some degree of competence. Only after competence is attained in several related areas should an exclusive emphasis be placed on practice in weak areas only.