1,2,4-Oxadiazoles and 4,5-dihydro-1,2,4-oxadiazoles are an interesting class of compounds because of their pharmacological and other properties. Although short descriptions of these compounds under mass spectrometric conditions exist, we thought it worthwhile to write a comprehensive review of this class of compounds. Therefore, this revision deals first with 1,2,4-oxadiazoles and then 4,5-dihydro-1,2,4-oxadiazoles from 1964 until the year 2002. Much work on these compounds has been described under electron impact conditions and only a little under chemical ionization (CI).