The Marcomanni were a Germanic tribal confederation who eventually came to live in a powerful kingdom north of the Danube, somewhere in the region near modern Bohemia, during the peak of power of the nearby Roman Empire. According to Tacitus and Strabo they were Suebian.
The Marcomanni first appear in historical records as confederates of the Suebi of Ariovistus fighting against Julius Caesar in Gaul (modern France), having crossed the Rhine from present-day southern Germany. The exact position of their lands at this time is not known. The fact that their name existed before the Romans had territory near the Danube or Rhine raises the question of which border they lived near in order to explain their name. Their name may echo an earlier demarcation between the northern Germanic tribes of the Jastorf cultural circle, and those of the Celtic maximum expansion during the earlier and later Iron Age of La Tene dominance throughout Europe, that from findings in the archaeological record pressed North through with some influence as far as into Jutland, but mostly remained separated South and settled on Oppidas over today Thuringia and Saxony along the Hercynian forest, intrinsically connected to the major trade roads that went into the more evolved centers of Bohemia, Moravia and Silesia all still Celtic regions then. It has as been suggested that they may have lived near the conjunction of Rhine and Main river, at the areas formerly inhabited but left deserted by the Helvetii and Taurisci. However the historian Florus reports that Drusus erected a mound of their spoils during his campaign of 12-9 BC, after defeating the Tencteri and Chatti, and before next turning to Cherusci, Suevi, and Sicambri, suggesting that they were not close to any obvious border at the time.
According to the accounts of Tacitus (Germ. 42), Paterculus (2.108), Pliny the elder, and Strabo (vii. p. 290) they eventually moved into the large area previously occupied by the Boii, specifically in a region already called Baiohaemum, where their allies and fellow Suevi the Quadi lived. This was described as being within the Hercynian forest and was possibly in the region of modern Bohemia, although this is not certain. By 6 BC, their king, Maroboduus, had established a powerful kingdom there that Augustus perceived as a threat to Rome. Before he could act, however, the revolt in Illyria intervened. Eventually Maroboduus was deposed and exiled by Catualda (AD 19). Catualda was in turn deposed by Vibilius of the Hermunduri the same year, and succeeded by the Quadian Vannius. Around 50 AD, Vannius was himself also deposed by Vibilius, in coordination with his nephews Vangio and Sido.
Tacitus, in the late 1st century mentions (Germania I.42) the Marcomanni as being under kings appointed by Rome. 
In the 2nd century AD, the Marcomanni entered into a confederation with other peoples including the Quadi, Vandals, and Sarmatians, against the Roman Empire. This was probably driven by movements of larger tribes, like the Goths. According to the historian Eutropius, the forces of the emperor, Marcus Aurelius, battled against the Marcomannic confederation for three years at the fortress of Carnuntum in Pannonia. Eutropius compared the war, and Aurelius's success against the Marcomanni and their allies, to the Punic Wars. The comparison was apt in that this war marked a turning point and had significant Roman defeats; it caused the death of two Praetorian Guard commanders. The war began in 166, when the Marcomanni overwhelmed the defences between Vindobona and Carnuntum, penetrated along the border between the provinces of Pannonia and Noricum, laid waste to Flavia Solva, and could be stopped only shortly before reaching Aquileia on the Adriatic Sea. The war lasted until Aurelius's death in 180. It would prove to be only a limited success for Rome; the Danube river remained as the frontier of the empire until the final fall of the West.
The Christianisation of the Marcomanni, at least into a Roman orthodox form of Christianity, seems to have occurred under their queen, Fritigil (wife of an unnamed king) in the mid fourth century. She corresponded with Ambrose of Milan to bring about the conversion. This was the last clear evidence of the Marcomanni having a polity. It was possibly on the Roman side of the Danube by this time. Soon after, the Pannonian and Danubian area went into a long period of turmoil.
After crossing the Rhine in 406 and the Pyrenees in 409, a group of Suevi, who had migrated together with Vandals and Alans, established themselves in the Roman province of Gallaecia (modern Galicia and northern Portugal), where they were considered foederati and founded the Suebi Kingdom of Gallaecia. These Suevi were probably a mix of Suevian groups from the area north of Danube and Pannonian basin such as the Marcomanni, Quadi and Buri.
The Danubian area meanwhile became the core of Attila the Hun's empire, and within it there seem to have been many Suebians. One group of them managed to reform into an independent group after the Battle of Nedao in 454, like many other groups who emerged from Attila's confederation. These Suevi eventually came into conflict with the Ostrogoths, who had been on the losing side at Nadao.
Jordanes, the historian of the Goths, reported (Getica 280) that after the Battle of Bolia, the Ostrogoths attacked the Suevi (ruled by a man named Hunimund, who also seems to have led an attack on Passau) by crossing the Danube when frozen, and going into a high Alpine area held by the confederates of the Suevi at this time, the Alamanni. (He said that several streams start in this area which enter the Danube with a loudly.) The region held by these Suevi was described as having Bavarians to the west, Franks to the east, Burgundians to the south, and Thuringians to the north. The text seems to indicate that these Suevi had moved into the Alamannic area but that these specific Suevi were seen as distinct from both Alamanni and Bavarians. This was also the first mention of Bavarians and they are also often proposed to have had Marcomanni in their ancestry.
According to historians such as Herwig Wolfram:
There is a runic alphabet called the Marcomannic runes, but they are not believed to be related to the Marcomannic people.