  Plasma boundary layers are susceptible to electrostatic instabilities driven
by ion flows in presheaths and, when present, these instabilities can influence
transport. In plasmas with a single species of positive ion, ion-acoustic
instabilities are expected under conditions of low pressure and large
electron-to-ion temperature ratio ($T_e/T_i \gg 1$). In plasmas with two
species of positive ions, ion-ion two-stream instabilities can also be excited.
The stability phase-space is characterized using the Penrose criterion and
approximate linear dispersion relations. Predictions for how these
instabilities affect ion and electron transport in presheaths, including rapid
thermalization due to instability-enhanced collisions and an
instability-enhanced ion-ion friction force, are also briefly reviewed. Recent
experimental tests of these predictions are discussed along with research needs
required for further validation. The calculated stability boundaries provide a
guide to determine the experimental conditions at which these effects can be
expected.
