The　lightning　overvoltage　and　switching　overvoltage　in　power　systems　are　both　impulse　voltages　with　short　duration　time　and　large　change　rate,　under　which　the　air　gap　breakdown　characteristics　are　different　from　those　under　steady　voltages.　This　chapter　firstly　introduces　the　air　gap　breakdown　characteristics　under　lightning　and　switching　impulse　voltages.　Then,　the　air　insulation　prediction　model　is　used　to　predict　the　switching　impulse　discharge　voltages,　lightning　impulse　discharge　voltages　and　volt-time　characteristic　curves　of　air　gaps　with　different　geometries　and　under　different　voltage　waveforms.　©　Springer　Nature　Singapore　Pte　Ltd.　and　Science　Press,　Beijing　2019.