Climate variability, both in space and in time, has a significant impact on nearly every aspect of the Critical Zone. The primary focus of this chapter is on soil moisture because it is an important component of the water balance in the Critical Zone and it is a key parameter that influences land-atmosphere interactions in the climate system by modifying energy and water fluxes in the boundary layer. Soil moisture plays an integrative role because it directly influences atmospheric, geomorphic, hydrologic, and biologic processes. This chapter describes how, and why, the coupling between land and atmosphere varies over time and space. An improved understanding of land-atmosphere interactions enhances our ability to explain and predict climate variability at local to global scales.