Joke TORBEYNS, Bert DE SMEDT, Greet PETERS, Pol GHESQUIERE, and Lieven VERSCHAFFEL, K.U.Leuven (Belgium) Indirect Addition: Theoretical, Methodological and Educational Considerations The development of fundamentally important arithmetic principles related to the four basic operations, and of arithmetic strategies that are based on these principles, is an intriguing and important element of psychological, mathematical and math educational research. As far as addition and subtraction are concerned, we have, for instance, the following principles: (a) the commutativity principle, which says that the order of the addends is irrelevant to their sum (a + b = b + a); (b) the principle prescribing that if nothing is added to or removed from a collection its cardinal value remains unchanged (a + 0 = 0; a - 0 = a); (c) the principle that adding an amount to a collection can be undone by subtracting the same amount and vice versa (a + b - b = a or a - b + b = a); and (d) the principle that if a + b = c, then c-b = a or c - a = b. Previous theorizing and research shows that understanding these principles plays an important role in children’s construction of the additive composition of number and in additive reasoning. Moreover, the implicit or explicit application of these principles can also considerably facilitate people’s arithmetic performance by eliminating computational effort and increasing solution efficiency (Baroody, Torbeyns, & Verschaffel, 2009). For example, the first principle underlies the well-known computation shortcut for solving additions starting with the smaller given number (like 2 + 9 or 4 + 58), that consists of reversing the order of operands and adding the smaller addend to the larger one. The fourth principle underlies the computation shortcut for solving subtractions involving a small difference between the two integers (like 11 - 9 or 61 - 59), by determining how much has to be added to the smaller integer to make the larger one. Whereas the first three above- mentioned principles and their accompanying computational shortcut strategies have already received a great amount of research attention (Verschaffel, Greer, & De Corte, 2007), the fourth principle has not. In this contribution, we will present a series of closely related studies in the domain of elementary subtraction that we have done so far on this fourth principle and its accompanying computational shortcut, namely indirect addition (IA). We will use the term direct subtraction (DS) for the more common straightforward strategy for doing subtraction whereby the smaller number is directly taken away from the smaller one. Use of IA in Young Adults