Épée turned his attention toward charitable services for the poor, and on one foray into the slums of Paris he had a chance encounter with two young deaf sisters who communicated using a sign language. Épée decided to dedicate himself to the education and salvation of the deaf, and in 1760 he founded a shelter which he ran with his own private income. In line with emerging philosophical thought of the time, Épée came to believe that deaf people were capable of language, and concluded that they should be able to receive the sacraments and thus avoid going to hell. He began to develop a system of instruction of the French language and religion. In the early 1760s, his shelter became the world's first free school for the deaf, open to the public.
Though Épée's original interest was in religious education, his public advocacy and development of a kind of "Signed French" enabled deaf people to legally defend themselves in court for the first time.
Abbé de l'Épée died at the beginning of the French Revolution in (1789), and his tomb is in the Saint Roch church in Paris. Two years after his death, the National Assembly recognised him as a "Benefactor of Humanity" and declared that deaf people had rights according to the Declaration of the Rights of Man and of the Citizen. In 1791, the " Institution Nationale des Sourds-Muets à Paris", which Épée had founded, began to receive government funding. It was later renamed the "Institut St. Jacques" and then renamed again to its present name: " Institut National de Jeunes Sourds de Paris". His methods of education have spread around the world, and the Abbé de l'Épée is seen today as one of the founding fathers of deaf education.
The Paris school and its daughter schools across Europe emphasised learning trades, such as printing, carpentry, masonry, gardening and tailoring. It was supported by the French government and people in part as a means to separate deaf people from their families, where they were poor dependents, and convert them into productive members of society.
After his death, he was succeeded by the Abbe Sicard who became the new head of the school.
In English, Épée's system has been known as "Methodical Signs" and "Old Signed French" but is perhaps better translated by the phrase "systematised signs". While Épée's system laid the philosophical groundwork for the later developments of Manually Coded Languages such as Signed English, it differed somewhat in execution. For example, the word croire ("believe") was signed using five separate signs — four with the meanings "know", "feel", "say", "not see" and one that marked the word as a verb (Lane, 1980:122). The word indéchiffrable ("unintelligible") was also produced with a chain of 5 signs: interior-understand-possible-adjective-not. However, like Manually Coded Languages, Épée's system was cumbersome and unnatural to deaf signers. A Deaf pupil of the school (and later teacher) Laurent Clerc wrote that the deaf never used the signes méthodiques for communication outside the classroom, preferring their own community language (French Sign Language).
Although Épée reportedly had great success with this educational method, his successes were questioned by critics who thought his students were aping his gestures rather than understanding the meaning.
Épée, to a lesser degree, also used speech and lip-reading with his pupils.
Some deaf schools in Germany and England that were contemporaries of the Abbé de l'Épée's Paris School used an 'oralist' approach emphasising speech and lip-reading in contrast to his belief in 'manualism'. Their methods were closely-guarded secrets and they saw Épée as a rival. The oralism vs. manualism debate still rages to this day. Oralism is sometimes called the 'German method' and manualism the 'French method' in reference to those times.
The Paris school still exists, though it now uses French Sign Language in class rather than Épée's methodical signs. Located in rue Saint-Jacques in Paris, it is one of four national deaf schools - the others being in Metz, Chambéry, and Bordeaux.
He is also wrongly cited as the inventor of the one-handed manual alphabet. Épée had actually been quite disdainful of the advocates of fingerspelling, and had himself used a different (two-handed) alphabet in instances where he felt it necessary to use one.