This chapter aims to present the measurement concept and evolution of wide-swath altimetry in a unified way because no such comprehensive description is available in the open literature. It describes the space–time characteristics and the magnitude of the water elevation signal for wide-swath altimetry's primary targets, together with the measurement requirements that have been endorsed by the ocean and hydrology communities. The chapter discusses measurement principles behind the concept and their evolution and presents a derivation of all the components of the error budget for wide-swath altimeters, which contains many more terms than for the traditional nadir altimeter. It explores the measurement physics changes due to the off-nadir viewing angles and the need to use Synthetic Aperture Radar and interferometry. The chapter examines the synthesis of these components into the design of a spaceborne mission. However, for both oceanography and hydrology, Surface Water Ocean Topography is considered as a first step.