[Excerpt] The main point of this chapter is to show that the different indices used in the mobility literature are not measures of the same underlying conceptual entity. In elementary statistics, students are taught that the mean and median are both measures of central tendency but they are different measures of central tendency; the variance and Gini coefficient are measures of dispersion but they are different measures of dispersion; and central tendency and dispersion are fundamentally different concepts from one another. In much the same way, this chapter maintains that the different mobility indices in common use are measuring fundamentally different mobility concepts from one another. It is in this sense that mobility really is multifaceted.