This paper reviews a novel and simple technique for measuring electron density using a plasma absorption probe (PAP). The PAP enables the measurement of local absolute electron density even when the probe surface is soiled with processing plasmas. The technique relies on the absorption of surface waves (SWs) resonantly excited around the probe head at critical frequencies which depend on the electron density. The PAP consists of a small antenna connected to a coaxial cable and is enclosed in a tube of dielectric constant ε_d inserted in a plasma of electron plasma frequency ω_p. A network analyzer feeds a rf signal to the antenna and displays the frequency dependence of the power absorption. A series of resonant absorptions is observed at frequencies slightly above the SW resonance frequency, ω_SW = ω_p/(1+ε_d)½, which allows us to determine the electron density. Typical examples of measured PAP data and future challenges are presented.