We examine when, why, and how people use category-based knowledge in determining how much they will like an object based on memories. We find that people rely on category-based knowledge when making liking evaluations of items from memory and rely on such knowledge more when items are typical of a category. We suggest that people rely on categories to fill in information that memories leave out and use typicality as a cue for the likelihood that category-based knowledge will be a good substitute for knowledge about an item. In 2 studies, using products and color patches as stimuli, we find that people rely on category liking evaluations in forming liking evaluations of specific items from memory and that they do so more when an items is typical (vs atypical) of that category. This work shows that category-based knowledge can play an important part in preference formation.