One way of describing the field of strategic management is to say that it is the study of how managers organize and position the firm to gain sustainable competitive advantage. By sustainable competitive advantage, we mean that the firm achieves superior returns on investments which do not erode in a short period of time. There are many ways to attain a competitive advantage (and even more ways to lose one). For example, a firm might exploit a first-mover advantage to enter into a particular market segment which is not large enough to accommodate more than one firm. Another strategy may rely on utilizing some unique set of resources or capabilities that other competitors do not have and cannot easily replicate. Most successful strategies (as one can see in the case of Wal-mart) rely on a combination of factors to create a sustainable competitive advantage.