The sound [ǝ] refers to one of the most complex and variable phenomena observed in the French speech stream. One of the most important features of [ǝ] is its ability to be both a sound and an absence of sound in almost the same context, i.e. without changing the informative content of the message. The specificity of French [ǝ] also lies in the fact that it is pronounced as a labialized vowel. As a result, three rounded vowels of the front row and middle rise are pronounced in an unstressed syllable: fluent [ǝ] and harmonized [œ] and [ø]. Taking into account the acoustic and articulatory similarity of these sounds and the law of least effort, the [ǝ] ability for complete reduction could be expected to be transferred to [œ] and [ø], especially in rapid speech. Do these facts indicate the French unstressed vocalism tendency towards simplification? How is this variability tracked by the speaker while understanding and generating speech? The article summarizes empirical data (acoustic, articulatory and perceptual) on the [ǝ] variability from phonetic, sociolinguistic and neuro-linguistic studies. It highlights major challenges and open issues that should be addressed in further research.