In Hermeticism 
Eliphas Levi (1810–1875), one of the first modern ceremonial magicians and inspiration for the Hermetic Order of the Golden Dawn, discussed the Great Work at length, expanding it from the purely alchemical towards the more spiritual:
Furthermore, there exists in nature a force which is immeasurably more powerful than steam, and by means of which a single man, who knows how to adapt and direct it, might upset and alter the face of the world. This force was known to the ancients; it consists in a universal agent having equilibrium for its supreme law, while its direction is concerned immediately with the great arcanum of transcendental magic... This agent...is precisely that which the adepts of the middle ages denominated the first matter of the Great Work. The Gnostics represented it as the fiery body of the Holy Spirit; it was the object of adoration in the secret rites of the Sabbath and the Temple, under the hieroglyphic figure of Baphomet or the Androgyne of Mendes.
He further defined it as such:
The Great Work is, before all things, the creation of man by himself, that is to say, the full and entire conquest of his faculties and his future; it is especially the perfect emancipation of his will.
In Thelema 
Within Thelema, the Great Work is generally defined as those spiritual practices leading to the mystical union of the Self and the All. Its founder, author and occultist Aleister Crowley, said of it in his book Magick Without Tears:
For each individual this Great Work may take different forms. Crowley described his own personal Great Work in the introduction to Magick (Book 4):
In my third year at Cambridge, I devoted myself consciously to the Great Work, understanding thereby the Work of becoming a Spiritual Being, free from the constraints, accidents, and deceptions of material existence.
Within the system of the A∴A∴ magical Order the Great Work of the Probationer Grade is considered to be the pursuit of self-knowledge to, as Crowley said in The Confessions of Aleister Crowley, "obtain the knowledge of the nature and powers of my own being." However, Crowley continues, the Great Work should also be something that is integrated into the daily life of all:
I insist that in private life men should not admit their passions to be an end, indulging them and so degrading themselves to the level of the other animals, or suppressing them and creating neuroses. I insist that every thought, word and deed should be consciously devoted to the service of the Great Work. 'Whatsoever ye do, whether ye eat or drink, do all to the glory of God'.
Although Crowley often discussed the idea of "succeeding" or "accomplishing" in the Great Work, he also recognized that the process is ongoing. From his Little Essays Toward Truth:
The Quest of the Holy Grail, the Search for the Stone of the Philosophers—by whatever name we choose to call the Great Work—is therefore endless. Success only opens up new avenues of brilliant possibility. Yea, verily, and Amen! the task is tireless and its joys without bounds; for the whole Universe, and all that in it is, what is it but the infinite playground of the Crowned and Conquering Child, of the insatiable, the innocent, the ever-rejoicing Heir of Space and Eternity, whose name is MAN?
The term also appears in the Benediction at the end of Crowley's Gnostic Mass, where the Priest blesses the congregation with the words:
- Redgrove, Herbert Stanley, Alchemy: Ancient and Modern, Section 43: Bernard Trévisan, Copyright 1999, by the Rector and Visitors of the University of Virginia
- Lévi, Éliphas (1968). Transcendental Magic: its Doctrine and Ritual. Arthur Edward Waite (trans.) ([Rev. ed.] ed.). London: Rider.
- Crowley, Aleister. Magick Without Tears, "Letter C." New Falcon Publications, 1991. ISBN 1-56184-018-1
- Crowley, Aleister; Mary. Desti, Leila. Waddell (2004). Magick:Liber ABA, Book 4, Parts I-IV. Hymenaeus. Beta (ed.). York Beach, Maine: Samuel Weiser. ISBN 0-87728-919-0 9780877289197 Check
- Crowley, Aleister. The Confessions of Aleister Crowley, Penguin, 1989. ISBN 978-0-14-019189-9
- Crowley, Aleister. Little Essays Toward Truth. "Man." New Falcon Publications, 1991. ISBN 1-56184-000-9