Tourists are not all the same, they have different pictures of their ideal vacation. Tourists are heterogeneous. Market segmentation is the strategic tool to account for heterogeneity among tourists by grouping them into market segments which include members similar to each other and dissimilar to members of other segments. Both tourism researchers and tourism industry use market segmentation widely to study opportunities for competitive advantage in the marketplace. This chapter explains the foundations of market segmentation, discusses alternative ways in which market segments can be formed, guides the reader through two practical examples, highlights methodological difficulties and points to milestone publications and recently published applications of market segmentation in the field of tourism.