All primates confine their travel, feeding, and social activity to a fixed area of their habitat called a home range (Bates 1970), which can vary in size from a few hectares to many square kilometers. When such an area, or a portion thereof, is defended against incursion by members of other social groups, as is the case with gibbons, it is designated a territory. Although the concepts of range use and territoriality are closely linked, it is useful from a theoretical standpoint to present them separately. In this chapter I will focus on the way ecological pressures influence gibbon ranging behavior, relying on the cost-benefit approach described in previous chapters. I will then turn to a discussion of gibbon territoriality in Chapter 6.