Hellenismos (Ἑλλήνισμος), also known as 'Dodekatheism', 'Hellenism', and 'Hellenic Polytheism' or 'Hellenic Reconstructionism,' is the modern practice of the ancient Hellenic (Greek) religion. It focusses on the worship of the Twelve Olympic Gods—Zeus, Hera, Athena, Hēphaistos, Apollon, Artemis, Demeter, Hestia, Hermes, Ares, Poseidon and Aphrodite—along with the entire pantheon of Hellenic Gods, known collectively as the Theoi. Most Hellenists honour other types of divinities, including nature spirits, Khthonic (or Underworld) Deities, and heroes. Both physical and spiritual ancestors are honoured. Hellenists tend to be hard polytheists, meaning they view all Gods as separate entities and not linked through portfolio.

'Hellenismos' is a word coined by the Emperor Julian in the 4th century AD. He was the first to refer to a ‘Hellenic way,’ which, at the time, encapsulated pre-Christian religion and the legacy of Greek philosophy and culture. Recently, Julian's 'Hellenismos' has been popularized as a way of referring to the revival of ancient Hellenic religion in the West. This presents some difficulties since, in modern Greek, it translates roughly as 'Hellenism, the Greek nation,' which is why some disagree with the term and would rather use 'Dodekatheism;' 'Worship of the Twelve Gods.'

Another problematic term is ‘Reconstructionism.’ We cannot begin to reconstruct the ancient religion because of our incomplete understanding of many practices, the absence of polis religion (i.e. group worship as practiced by inhabitants of a city) preventing the celebration of festivals in their traditional way, and the vast differences between the ancient and modern cultures, just to name a few. The best we can do is understand ancient practice based on scholarly research and to base our modern practice on the spirit of ancient practice. Religions, like all things, must adapt to the times, but preserve the essence of traditional, dynamic, and non-dogmatic practice. When the word ‘Reconstruction’ is used in regards to Hellenismos, it is mostly used in a Pagan context where it is used to differentiate between practices that are based on ancient ethnic religions and modern Pagan religions and traditions, most notably Wicca and related paths.

Hellenismos in its current—though ever-growing—form was established in the 1990's, although many members have practiced the ancient Hellenic religion for much longer than that. Over the last decade or so, two branches of Hellenismos have emerged: Traditional Hellenismos is a manner of following the spirit of ancient practice, where the practices of the ancient Hellenic peoples are understood and applied as much as possible in the modern day setting. This method requires knowledge of the ancient ways, but even more so, an understanding of it, so any modern adaption can be undertaken in the spirit of the ancient Hellenic practice. Anything else is Reformed Hellenismos and it often includes the incorporation of one or more belief systems or practices that are not strictly Hellenic—including the use of modern magic, the inclusion of 'patron Gods', or the appropriation of non-Hellenic ritual customs from other (often Pagan) religions.

The ancient Hellenes did not have a word for religion; the spiritual world and the tangible world were one and the same as Plotinus describes as late as the 3rd century CE. It recognizes and sanctifies natural cycles and the natural order of the kosmos. It is not a belief system and has no dogma but one of connecting with and understanding one's relationship to the real world of Nature. It is all about living the best life possible rather than a promise of the hereafter, about virtue rather than sin and redemption. 

Hellenists practice the religion of ancient Hellas in much the same spirit of the ancients: with reverence to the Gods, and with respect to the poets and philosophers. Hellenismos, as a way of life, is both mainstream and community-minded, and concerned with living a good and pious life. Today, the household is the seat of worship, not temples, and at its core is the relationship we build with the Theoi through sacrifice in ritual. This act of establishing and maintaining relationship with the Theoi is known as 'kharis'. There is no central 'ekklesia' (church or assembly) or hierarchal clergy, though some groups do offer training in that capacity. Individual worshipers are generally expected to perform their own rituals and learn about the religion and the Gods by reference to primary and secondary sources on ancient Greek religion and through personal experience with the Gods.

Furthering ones knowledge of the Hellenic Gods and religion, as well as all other areas in life is a valued part of Hellenismos. Studying as much as you can is encouraged. Knowledge—including knowledge of yourself—is essential. Ethics is at the very core of Hellenismos, and supports the heart of human life: arête, the act of living up to one's full potential. When one lives in the way of arête, they live their life ethically, consciously, and in happiness. The ancient Hellenes had many guidelines for this ethical framework. As such, Hellenismos is known for its highly developed ethical system, derived from ancient sources, especially the Delphic Maxims. Hellenismos encourages the practitioner to take control of their life; to become an active participant in it. Arête, for Hellenists, should become a way of life.

Christiane Sourvinou-Inwood writes in Oxford Readings in Greek Religion, Oxford 2000: "Greek religion is, above all, a way of articulating the world, of structuring chaos and making it intelligible: it is a model articulating a cosmic order guaranteed by a divine order which also (in complex ways) grounds human order, perceived to be incarnated above all in the properly ordered and pious polis, and providing certain rules and prescription of behaviour,  especially towards the divine through cult, but also towards the human world." She also writes that it "...was not a dogmatic schema demanding faith, but an open system proposing certain articulations of the world and transversed by the fundamental Greek category of unknowability."

Hellenismos from a historic standpoint was a communal religion, practiced in groups which strengthened solidarity, community, and kharis which was applied to the group or city as a whole. As modern practitioners are often few and far between, many on-line groups have arisen throughout the years to at least foster a sense of community where there isn't one physically. Even then, household rituals are often performed in the name of members of the—extended—household to grant kharis even to non-believers close to the practitioner.

Hellenismos is a religion focussed on community and family on the one hand, and personal excellence on the other, where the worship and love of the Theoi serves as an overarching ideal. It requires a sharp mind and open thought, as well as the willingness to adopt a practice that is both devout and fully ingrained into daily life. Because of these prerequisites, it is not an easy religion to adhere to, but it is a very fulfilling and rewarding one if practiced with arête.