This critical review outlines the current state-of-the-art research on the reversibly switchable wettability of surface brought about by external stimuli and the exchange of counterions. Chemical composition and surface topography are the two key factors in the wettability of solid substrates. Applying external stimuli and exchanging counterions of ionic liquids and polyelectrolyte films are valuable approaches for rendering the change in surface chemistry and/or topography, and for driving the transition between hydrophilicity and hydrophobicity of surfaces. Through the combination of stimuli-responsive films and micro-/nanostructural surfaces, smart surfaces with reversible switching between superhydrophobicity and superhydrophilicity have been achieved. As an important advancement in reversibly switchable wettability, this review briefly introduces ionic liquids (ILs) as on-off systems to obtain reversibly switchable wettability and then discusses in more detail the methods to induce the reversibly switchable wettability of surfaces modified by ILs, additives, or thin films. In addition to reversibly switchable wettability mechanisms, open problems and potential solutions are discussed (157 references).