In Greek mythology, the underworld or Hades (Ancient Greek: ᾍδης, romanized: Háidēs) is a distinct realm (one of the three realms that make up the cosmos) where an individual goes after death. The earliest idea of afterlife in Greek myth is that, at the moment of death, an individual's essence (psyche) is separated from the corpse and transported to the underworld. In early mythology (e.g., Homer's Iliad and Odyssey) the dead were indiscriminately grouped together and led a shadowy post-existence; however, in later mythology (e.g., Platonic philosophy) elements of post-mortem judgment began to emerge with good and bad people being separated (both spatially and with regard to treatment).

The underworld itself—commonly referred to as Hades, after its patron god, but also known by various metonyms—is described as being located at the periphery of the earth, either associated with the outer limits of the ocean (i.e., Oceanus, again also a god) or beneath the earth. Darkness and a lack of sunlight are common features associated with the underworld and, in this way, provide a direct contrast to both the 'normality' of the land of the living (where the sun shines) and also with the brightness associated with Mount Olympus (the realm of the gods). The underworld is also considered to be an invisible realm, which is understood both in relation to the permanent state of darkness but also a potential etymological link with Hades as the 'unseen place'. The underworld is made solely for the dead and so mortals do not enter it – with only a few heroic exceptions (who undertook a mythical catabasis: Heracles, Theseus, Orpheus, possibly also Odysseus, and in later Roman depictions Aeneas).

The deceased could enter the underworld through various routes, but perhaps the most common depiction is that of the ferryman Charon to take them across the river. This was a particularly common motif on Athenian white ground lekythoi (funerary vases) of fifth century BCE and it is difficult to date this figure much earlier than the 6th century BCE (perhaps the latest of Hades' inhabitants to 'appear'). While Charon doesn't feature in the earliest mythical sources, there was still a superstition that the unburied couldn't cross over until they receive a proper burial (the most famous examples being Patroclus and Hector in the Iliad). Alternatively, Hermes Psychopompos could also be relied upon to lead the deceased to the underworld and appears first in Homer's Odyssey book 24 (also a common motif on white ground lekythoi). Hades was well known for its gates (the πύλαι Ἀίδαο: Hom. Il. 5.646, 9.312, 23.71, Od. 14.156), with one of Hades's (the god) epithets being the "gate fastener" (πυλάρτης: Hom. Il. 8.367, 13.415, Od. 11.277).

Rivers are a fundamental part of the topography of the underworld and are found in the earliest source materials: In Homer's Iliad, the "ghost" of Patroclus makes specific mention of gates and a river (unnamed) in Hades; in Homer's Odyssey, the "ghost" of Odysseus's mother, Anticlea, describes there being many "great rivers and appalling streams", and reference is made to at least four specific rivers. H. A. Guerber assumed that the rivers where Charon sailed mirrored the sky in Greco-Roman thought. In the wider mythological tradition, however, there are multiple bodies of water that are associated with the underworld (varying in number and combination depending on the source), the names of which can be understood to reflect specific associations with death.

The Asphodel Meadows is the location in the underworld where the majority of the deceased dwell. The name appears as far back as Homer's Odyssey (11.359), where it features in Odysseus' survey of the underworld (technically referred to here as the field of asphodel: ἀσφοδελὸς λειμών). It is unclear exactly what the ancients understood this "field" to be, with scholars divided between either associating it with the flower asphodel (genus, Asphodelus L.) or with a field of ash (basing this on the etymological construction of σφοδελὸς > σποδός, "ash").

The Elysium (also referred to as the Elysian Fields) was a utopian, paradisiacal afterlife reserved for specially distinguished individuals. The Elysian Fields are first referenced in Homer's Odyssey Book 4 where Menelaus is promised that he will go there instead of dying (and so distinguishing it from the afterlife proper): it is described as being located at the edges of the earth (the peirata) and is where life is "easiest for men". However, Menelaus does not achieve this fate due to anything he has done during his lifetime but rather because he is Zeus' son-in-law (being married to Helen). In Hesiod's Works and Days, however, this is a paradise that heroes could attain. Eventually, as concepts of the afterlife broadened and became more "democratic", the generally righteous could be sent to the Elysian Fields after being judged by the underworld judges, Rhadamanthus and Minos.

By Hesiod's time, the Elysium would also be known as the Fortunate Isles or the Isles of the Blessed. The isles, which were sometimes treated as a geographical location on Earth, would become known as a place of reward in the underworld for those who were judged exceptionally pure.

Erebus is the personification of darkness, particularly that of the underworld. The name "Erebus" is often used by ancient authors to refer either to the underworld, or to the subterranean region through which souls of the dead travel to reach the underworld.