A large literature has documented that comparison and contrast lead to better performance in a variety of tasks. However, studies of comparison and contrast present contradictory conclusions as to when and how these processes benefit learners. Across four studies, we examined how the specifics of the comparison and contrast task affect performance by systematically manipulating the feature variation and category structure in a category extension task performed by 3-year-old and 4-year-old children. Studies 1 (n = 48, M = 42.6 months) and 2 (n = 48, M = 42. 4 months) investigated comparison and contrast with high-density categories. Studies 3A (n = 60, M = 43.47 months), 3B (n = 48, M = 53. 2 months) and 4 (n = 48, M = 53.7 months) investigated comparison and contrast with low-density categories. Results indicated both category structure and feature variation affect the efficacy of comparison and contrast. Copyright © 2012 John Wiley & Sons, Ltd.