Radioisotopes (particularly ²² Na, ²⁴ Na, ⁴² K, and ⁸⁶ Rb) have been used for many decades to trace the fluxes and accumulation of sodium and potassium ions in plant tissues. In this article, standard procedures for the tracing of ion fluxes are described, with emphasis on special problems encountered when examining K⁺ and Na⁺ transport under salinity conditions. We focus in particular on unidirectional influx measurements, while also providing a brief introduction to compartmental analysis by tracer efflux.