Marduk rose from an obscure deity in the third millennium BCE to become one of the most important gods and the head of the Mesopotamian pantheon in the first millennium. He was the patron god of the city of Babylon, where his temple tower, the ziggurat Etemenanki ("Temple (that is) the foundation of the heavens and the earth") served as the model for the famous "tower of Babel." In the first millennium, he was often referred to as Bel, the Akkadian word for "Lord."
Marduk is one of the most complex gods in ancient Mesopotamia and the short contribution here cannot do justice to this important deity. A comprehensive, monographic treatment of Marduk is still lacking (for now see Sommerfeld 1982, which excludes evidence from the first millennium BCE; also see Oshima 2011, who focuses on Akkadian prayers to Marduk).
Marduk's origins and original functions are obscure. He is associated with incantations already in the Old Babylonian period (Sommerfeld 1987-90: 368). Whether this was the reason for or result of a syncretism with the god Asalluhi is unclear (Sommerfeld 1982: 13-18). More recently Sommerfeld (1987-90: 368) cautiously indicated that his function as a god of incantations may have been original to Marduk. A different suggestion holds that Marduk was deliberately syncretised with Asalluhi in order to give Marduk a strong association with the city of Eridu and the god Enki/Ea, a powerful deity who did not belong to the original pantheon of Nippur (Abusch 1999: 543-4).
At the same time Marduk is mainly known as the patron god of the city of Babylon, and it has often been suggested that Marduk's religious importance increased with the city's growing political influence (e.g., George 1992: 248-9; Oshima 2007: 348). In the first millennium, Marduk is identified with Jupiter (Abusch 1999: 542).
One of the best-known literary texts from ancient Mesopotamia describes Marduk's dramatic rise to power: Assyriologists refer to this composition by its ancient title Enūma eliš, Akkadian for "When on high" (for recent translations see Foster 2005: 436-86; Lambert 2008: 37-59). It is often called "The Babylonian Epic of Creation," which is rather a misnomer as the main focus of the story is the elevation of Marduk to the head of the pantheon, for which the creation story is only a vehicle (Michalowski 1990; Katz 2011). In this narrative, the god Marduk battles the goddess Tiamat, the deified ocean, often seen to represent a female principle, whereas Marduk stands for the male principle. Marduk is victorious, kills Tiamat, and creates the world from her body. In gratitude the other gods then bestow 50 names upon Marduk and select him to be their head. The number 50 is significant, because it was previously associated with the god Enlil, the former head of the pantheon, who was now replaced by Marduk. This replacement of Enlil is already foreshadowed in the prologue to the famous Code of Hammurabi, a collection of "laws," issued by Hammurabi (r. 1792-1750 BCE), the most famous king of the first dynasty of Babylon. In the prologue, Hammurabi mentions that the gods Anu and Enlil determined for Marduk to receive the "Enlil-ship" (stewardship) of all the people, and with this elevated him into the highest echelons of the Mesopotamian pantheon.
Another important literary text offers a different perspective on Marduk. The composition, one of the most intricate literary texts from ancient Mesopotamia, is often classified as "wisdom literature," and ill-defined and problematic category of Akkadian literature (van Dijk 1953; Lambert 1996 ; Buccellati 1981; Denning Bolle 1992; various contributions in Clifford 2007). Assyriologists refer to this poem as Ludlul bēl nēmeqi "Let me praise the Lord of Wisdom," after its first line, or alternatively as "The Poem of the Righteous Sufferer" (Foster 2005: 392-409; also see the more recent but problematic edition by Annus and Lenzi 2010). The literary composition, which consists of four tablets of 120 lines each, begins with a 40-line hymnic praise of Marduk, in which his dual nature is described in complex poetic wording: Marduk is powerful, both good and evil, just as he can help humanity, he can also destroy people. The story then launches into a first-person narrative, in which the hero tells us of his continued misfortunes. It is this element that has often been compared to the Biblical story of Job. In the end the sufferer is saved by Marduk and ends the poem by praising the god once more. In contrast to Enūma eliš, the "Poem of the Righteous Sufferer" offers insights into personal relationships with Marduk. The highly complicated structure and unusual poetic language make this poem part of an elite and learned discourse.
Some scholars have described Marduk as a storm god or a god of vegetation (Abusch 1999: 544 with reference to Thorkild Jacobsen). There is no evidence for this assumption, other than highly problematical etymological speculations (see below).
Marduk was already syncretised with the gods Asalluhi (Sommerfeld 1987-90: 362; Richter 2004: 14-15) and Tutu (the patron deity of the city of Borsippa) (Sommerfeld 1982: 37) in the Old Babylonian period, although in some Old Babylonian sources Asalluhi and Marduk were still understood as separate deities (Richter 2004: 15 and n.57). The syncretism with Asalluhi is mentioned in a Sumerian literary letter to the goddess Ninisinna (Brisch 2007: 142-56), in which Asalluhi is described as the "king of Babylon."
Although the spelling of Marduk's name (see below) appears to affiliate him with the sun god Utu/Šamaš, there is no evidence that he was ever considered to be the sun god's son (Sommerfeld 1982: 11). On the contrary, tradition identifies Marduk as Enki/Ea's son, clearly affiliating him with the pantheon of Eridu.
Marduk's wife was the goddess Ṣarpanitum (Sommerfeld 1987-90: 362). The god Nabu, who was first Marduk's minister, later became identified as his son and then became his co-regent at the helm of the Babylonian pantheon.
Marduk's main cult place was the city of Babylon, which became the religious centre of Mesopotamia during the second and first millennia BCE (see various contributions in Cancik-Kirschbaum et al. 2011). Marduk's main temples were located in Babylon itself: the most famous was the ziggurat Etemenanki ("Temple (that is) the foundation of Heavens and Earth"), his temple tower in Babylon, which served as a model for the Biblical "Tower of Babel." The temple where Marduk was worshipped was called the Esagil (literally: "Temple whose top is raised," or perhaps better "Proud/Honoured Temple"). In addition, there was the akītu-house at Babylon, where the New Year's festival was celebrated. The akītu-house was located just outside the sacred district of Babylon (Pongratz-Leisten 1994). A ritual text dating to the Parthian period describes how Enūma eliš was recited in front of Marduk's statue during the New Year's festival, which also involved a ritual slapping of the king (Black 1981; Smith 1982; Kuhrt 1987; see also BRM 4, 7; TCL 0, 39; TCL 6, 40).
It is unclear whether Marduk is already attested in the Early Dynastic period (Sommerfeld 1987-90: 363). While some scholars have suggested that the divine name on an Early Dynastic inscription should be read dAMAR.UD (Lambert 1984: 8; Oshima 2007: 356 n.2), Sommerfeld (1982: 19-21; 1987-90: 362-3): has argued that it should be read dAMAR.DÙG and therefore cannot (or only tentatively) be identified with Marduk.
The Old Babylonian period offers more evidence for Marduk's growing popularity (Sommerfeld 1982), which remains restricted to northern and middle Babylonia (the evidence for his cult in southern Babylonian is rather meagre). Because the Old Babylonian archives from the city of Babylon are still largely unpublished (see Pedersen's  catalogue of texts from Babylon), the beginnings of Marduk's unprecedented rise to power in the capital of Hammurabi's empire are still poorly understood and will have to await further studies.
In the second half of the second millennium BCE, Marduk was often invoked by rulers of the Kassite dynasty, who had made Babylon their capital (Sommerfeld 1987-90: 265). With the Elamite invasion of Babylonia, which ended the Kassite dynasty, the divine statue of Marduk was abducted to Elam (Iran) together with other Babylonian cultural goods. It was not until Nebuchadnezzar I of the Second Dynasty of Isin that Marduk's statue could be retrieved and returned to Babylon in triumph. The "Babylonian Epic of Creation" (see above) was probably either composed or at least popularised during this period.
Due to tensions between Assyria and Babylonia during the Neo-Assyrian period, Marduk's cult and the city of Babylon often became the focus of Assyrian kings, both in positive and negative ways. It was not until the Neo-Babylonian period that Babylon and Marduk were at the apex: Nebukadnezzar II (r. 604-562 BCE) rebuilt the city of Babylon and with it the sacred district of Babylon. Most of the renowned architecture that was excavated in Babylon, such as the famous Ištar Gate with the glazed brick reliefs showing the symbol animals of Ištar and Marduk, dates to this period.
The early iconography of Marduk is unclear due to lack of sources. Already in the Old Babylonian period, the spade became Marduk's symbol (Rittig 1987-90: 372). The animal that often represents Marduk is the mušḫuššu, the "snake-dragon," which is frequently represented on the glazed brick reliefs from Babylon. There are very few anthropomorphic depictions of Marduk, most of them can be found on cylinder seals.
The etymology of Marduk's name is controversial (Sommerfeld 1987-90: 361-2). It is difficult to determine whether the logographic writing of his name dAMAR.UD, Sumerian for "calf of the sun/sun-god," is in any way significant or not (Lambert 1984: 7-8; Sommerfeld 1987-90: 361-2). The suggestion to translate this spelling as "calf of the storm" (Abusch 1999: 543) should probably be rejected as there is no evidence for Marduk originally having been a storm god nor is there evidence for his association with the storm god Iškur/Adad (see above).
Nicole Brisch, 'Marduk (god)', Ancient Mesopotamian Gods and Goddesses, Oracc and the UK Higher Education Academy, 2016 [http://oracc.museum.upenn.edu/amgg/listofdeities/marduk/]