Unity. The world of Islam is often regarded as one of great unity, which is attributed to a common faith. To the degree that Muslims quite consciously think of themselves as belonging to a single worldwide community and look back on the career of the Prophet Muhammad and the text of the divine revelations he received between 610 and 632 as a common heritage, the idea of unity has substance. Yet, there are also many differences among Muslim peoples and the regions they inhabit. This chapter covers aspects of unity and disunity that are not directly related to religious belief. Since it is not uncommon for Muslims and non-Muslims alike to feel that there is something distinctive about living or traveling in the Islamic world, this chapter explores possible historical roots for that feeling in the areas of communication and transportation and the degree to which that feeling was expressed...