Accessibility is determined by the distributions of supply and demand and how they are connected in space, and thus is a classic issue for location analysis well suited for GIS to address. This chapter focuses on how spatial accessibility is measured by GIS-based methods. Section 5.1 overviews the issues on accessibility, followed by two GIS-based methods for deﬁning spatial accessibility: the ﬂoating catchment area method in Section 5.2 and the gravity-based method in Section 5.3. Section 5.4 illustrates how the two methods are implemented in a case study of measuring access to primary care physicians in the Chicago region. The chapter is concluded with extended discussion and a brief summary.