The twenty books containing Seneca's 120 Epistulae ad Lucilium, composed between 62 and 65 AD when the philosopher had already abandoned politics, have survived in two different groups. The first contains letters n°1-88 which can also be distinguished into two sections (letters n°1-52 and n° 53-88); the second instead contains letters n°89-124. It is also possible that in Antiquity a third corpus of Seneca's letters was in circulation as Gellius quotes a passage of book XXIII unknown to us.

The addressee Lucilius, although of humble origin, had managed to enter the equestrian order and obtain in 63-64 AD some important administrative offices in Sicily as well as engage himself in literary and philosophical achievements. Seneca in these same years had dedicated him many works (e.g. De Providentia, Naturales quaestiones and the lost Moralis philosophiae libri).

In the Epistulae his main message is the importance of keeping political engagements at bay and using one's freedom of mind (otium) in the best possible way. Seneca clearly draws from his own personal experience.

Lucilius' role is twofold: he is both the addressee of Seneca's teaching and the instrument of Seneca's own practice in the art of living and dying properly. At first Seneca - possibly foreseeing his own end - praises suicide as the best possible way to demonstrate one's contempt of death , then as the last resort against physical decay and finally as the only possible choice for the sage. It is not clear if these letters were really intended for Lucilius or simply as a literary exercise destined for publication. In any case, Seneca writes with the consciousness of who willingly introduces into the Latin world a new literary and philosophical genre. His correspondence where letters born out of personal matters develop into complex philosophical treatises is in fact the first of this sort in the Western world.

Naturally the letter form is particularly congenial to his work as it reflects the simplicity of everyday language concerning commonplace matters and his style, although refined, makes use at times of family life expressions.