Elvira de Hidalgo, was Maria Callas' most important voice teacher probably had the greatest influence on Maria's technique and career. By the time Maria came to study with her at the Athens Conservatoire (during the second world war), de Hidalgo had already had a significant singing career as a coloratura soprano. It is interesting to note that the life of de Hidalgo encompasses the entire life and career of Callas; at the time of Callas' death, de Hidalgo was still alive and teaching and was interviewed on the subject of Maria. She must have been in her late eighties by this time (1977).

There are some recordings of de Hidalgo. Her "una voce poco fa" is typical of the singing of the excellently trained singers of the turn of the century. Her timbre, ease of production and great agility are all hallmarks of the kind of singing technique which such teachers as Mathilde Marchesi taught in the late 19th century. Clearly, Maria Callas received the rigorous and well defined training of this Italian school of singing from her teacher. Callas' immaculate ornamentation, smooth scales, skillful use of her vocal registers all reflect the training of this vocal method.

It seems likely that Maria Callas also gained deep insight into expression, use of words, bringing the character to life through the voice and the music from this teacher. It is known that Maria Callas attended the voice lessons of all her singing colleagues at the Athens Conservatoire, this is likely to have greatly informed her own style and artistry, which were second to none.