The Surface Water and Ocean Topography (SWOT) mission represents a fundamental transformation in satellite hydrology, advancing river monitoring from sparse and opportunistic observations to systematic reach-based measurement. SWOT’s Ka-band Radar Interferometer provides two-dimensional maps of water surface elevation, slope, and width at spatial resolutions of 50 to 100 m across a 120-km swath, with centimeter-scale vertical accuracy for rivers wider than approximately 50 to 100 m. This chapter situates SWOT within the broader evolution of satellite hydrology, tracing progress from early radar altimetry to multi-sensor integration, and introduces a multidimensional framework of river observability that encompasses detectability, measurability, reliability, and usability. Building on this framework, we propose a global typology of observability comprising width-dominated, seasonally variable, vegetation-limited, topographically constrained, and anthropogenically modified river systems, each characterized by predictable spatial and temporal patterns in data quality. We further evaluate the critical role of hydrographic infrastructure, with emphasis on the Surface Water and Ocean Topography River Database (SWORD), and the persistent challenge of reconciling SWOT observations with model-based river networks. Beyond advancing hydrological theory, SWOT delivers politically neutral and globally consistent datasets that support flood forecasting, water management, and monitoring of sustainability indicators in regions where in situ data are sparse. Looking forward, the long-term legacy of SWOT will depend on continuity of observations, harmonized hydrography, integration across multiple sensor frequencies, and equitable capacity development to ensure that its scientific and societal benefits are realized globally.