The cross-analysis of geographical environments and health generates many methodological challenges pertaining to the integration of daily mobility. This chapter references the methodological challenges around the issue of individual daily mobility when assessing environmental effects on health. To begin with, it introduces the conceptual and methodological importance of taking daily mobility into account when studying context effects on health through the concept of activity space. The chapter then discuss the different tools for collecting data, representations, the characterization and quantification of the activity space in health studies. It reviews the importance of the “selective daily mobility bias” or the impact of individual preferences in daily mobility. The chapter also addresses new challenges in terms of causal inference associated with the selective daily mobility bias.