Adding, subtracting, multiplying, and dividing with decimal numbers constitute a sizable portion of the elementary school mathematics curriculum. Unfortunately, these ideas are often taught(and learned) only on a surface level. There are a host of rules and procedures that can be the focus of instructional attention rather than the concepts behind those rules. We will focus on the concepts. If learners understand the concepts of arithmetic with decimal numbers, then even if they forget the rules they can generally figure them out again on their own. This is the type of understanding that we are aiming for.