Robert Siegler is Teresa Heinz Professor of Cognitive Psychology. His research focuses on children’s thinking, particularly their mathematical and scientific thinking. He has published more than 200 articles and chapters, written 9 books, and edited 5 others. His books have been translated into numerous languages, including Japanese, Chinese, Korean, German, French, Greek, Hebrew, Spanish, and Portuguese. Siegler’s research focuses on the development of problem solving and reasoning in general and on the more specific topics of how children learn mathematics and how theoretical understanding of mathematical development can be applied to improving the learning of children who have difficulty learning mathematics. The theoretically oriented research examines how children’s basic representations of numbers influence their ability to learn whole number arithmetic, fractions, and other aspects of mathematics. Among the topics examined within this work are how representations of numbers change with age and experience, types of mathematical experiences that are especially helpful in producing improvements, the strategies that children use to solve mathematical problems, why some children are more mathematically proficient than others, and how children discover new strategies. This research suggested that certain types of numerical board games would be especially helpful for improving young children’s mathematical understanding. Experimental tests of this prediction have yielded encouraging results; playing these board games yields large, rapid, and enduring gains in preschoolers’ and young elementary school children’s understanding of whole numbers. The gains are especially large with preschoolers from low-income backgrounds. More recently, Siegler’s research has produced similarly large games through computer games promoting learning of fractions.