Germanic tribe
The Frisii were an ancient tribe, living in the low-lying region between the Rhine–Meuse–Scheldt delta and the River Ems, sharing some cultural and linguistic elements with the neighbouring Celts. The newly formed marshlands were largely uninhabitated until the 6th or 5th centuries BC, when inland settlers started to colonize the area. As sea levels rose and flooding risks increased, the inhabitants learned to build their houses on village mounds or terps. The way of life and material culture of the Frisii hardly distinguished itself from the customs of the Chaucian tribes living farther east. The latter, however, were considered to be part of the Germanic tribal confederation.
During the 1st century BC, Romans took control of the Rhine delta but Frisii to the north of the river managed to maintain some level of independence. There was a lot of interaction, however, as Frisian and Chaucian mercenary bands enlisted in the Roman army and Roman traders established themselves north of the limes. There may have been Roman military outposts on Frisian territory. Some or all of the Frisii may have merged with Frankish and Saxon migrants in late Roman times, but they would retain a separate identity in Roman eyes until at least 296, when Frisian, Frankish and Chamavian groups were forcibly resettled as "laeti". Archaeological findings suggest that they may have been transported to Flanders and Southwestern England. 
The area where the original Frisii lived was largely deserted during the Migration Period, probably due to political instability and piracy, as well as climatic deterioration and frequent flooding caused by sea level rise. When changing environmental and political conditions made the region attractive again it was repopulated in the 5th century by Anglo-Saxon settlers from Northwestern Germany and Southwestern Denmark, who adopted the old name Frisii. These new 'Frisians' lived in the coastal fringe stretching roughly from present-day Bruges to Bremen, including many of the smaller offshore islands. They incorporated the remainer of indigenous groups that lived in the area and successfully conquered what would become their new homelands. Medieval and later accounts of 'Frisians' refer to these 'new Frisians' rather than to the ancient Frisii.
Description.
What little is known of the Frisii is provided by a few Roman accounts, most of them military. Pliny the Elder (AD 23–79) said their lands were forest-covered with tall trees growing up to the edge of the lakes.
They lived by agriculture and raising cattle. In the late 1st century the Romans referred to the 'Greater Frisii' as living to the east of the Lake Flevo, and the 'Lesser Frisii' to the west of it, so-called for their proportional power, and with the settlements of both stretching along the border of the Rhine to the ocean.
In his "Germania" Tacitus would describe all the Germanic peoples of the region as having elected kings with limited powers and influential military leaders who led by example rather than by authority. The people lived in spread-out settlements. He specifically noted the weakness of Germanic political hierarchies in reference to the Frisii, when he mentioned the (apparently Celtic) names of two kings of the 1st century Frisii and added that they were kings "as far as the Germans are under kings".
Early Roman accounts of war and raiding do not mention the Frisii as participants, though the neighboring Canninefates (to the west and southwest, in the delta) and Chauci (to the east) are named in that regard. The earliest mention of the Frisii tells of Drusus' 12 BC war against the Rhine Germans and the Chauci. The Romans did not attack them after devastating the lands of the Rhine Germans, but merely passed through their territory and along their coast in order to attack the Chauci. The account says that the Frisii were "won over", suggesting a Roman suzerainty was imposed, although the Romans never outright took over the lands of the Frisii.
Over the course of time the Frisii would provide Roman auxiliaries through treaty obligations, but the tribe would also appear in its own right in concert with other Germanic tribes, opposing the Romans. Accounts of wars therefore mention the Frisii on both sides of the conflict, though the actions of troops under treaty obligation must have been separate from the policies of indigenous groups.
Wars with the Romans.
The Frisii were little more than occasional and incidental players in Roman accounts of history, which focus on Roman actions that were of interest to Roman readers. As a consequence, references to them are disjointed and offer little useful information about them.
When Drusus brought Roman forces through Frisii lands in 12 BC and "won them over", he placed a moderate tax on them. However, a later Roman governor raised the requirements and exacted payment, at first decimating the herds of the Frisii, then confiscating their land, and finally taking wives and children into bondage. By AD 28 the Frisii had had enough. They hanged the Roman soldiers collecting the tax and forced the governor to flee to a Roman fort, which they then besieged. The propraetor of Germania Inferior, Lucius Apronius, raised the siege and attacked the Frisii, but was defeated at the Battle of Baduhenna Wood after suffering heavy losses. For whatever reason, the Romans did not seek revenge and the matter was closed. The prestige of the Frisii among the neighboring Germanic tribes was raised considerably after these events.
After their experiences with the predatory Roman governor and Lucius Apronius, the Frisii became disaffected towards Rome. In AD 47, a certain Gannascus of the Canninefates led the Frisii and the Chauci to rebel. They raided along the then-wealthy coast of Gallia Belgica. The Roman military commander, Corbulo, campaigned successfully against the Germanic tribes, For the Chauci and for the Frisii this meant Roman occupation, with the Romans specifying where they must live, with a fort built among them, and forcing a Roman-style senate, magistrates, and constitution upon them.
The Frisii are next mentioned in 54, when they occupied empty, Roman-controlled land near the Rhine, settling into houses and sowing and plowing fields. The Romans attempted to persuade them to leave, and even invited two Frisii kings to Rome to meet Nero, who ordered them to leave. The Frisii refused, whereupon a Roman military force coerced them, killing any who resisted.
In AD 69 the Batavi and other tribes rose against Roman rule in the Revolt of the Batavi, becoming a general uprising by all the Germans in the region, including the Frisii. Things went well for the Germans at first. One of the early leaders, Brinno of the Canninefates tribe, quickly defeated a Roman force of two cohorts and took their camp. The capable Civilis ultimately succeeded to leadership of the Germanic side and inflicted heavy casualties on the Romans, even besieging Roman strongholds such as Vetera. On the sea, a Roman flotilla was captured by a Germanic one. However, the war did not end well for the Germans. Led by Cerialis, the Romans ultimately forced a humiliating peace on the Batavi and stationed a legion on their territory.
In the course of the war, both the Frisii and the Chauci had auxiliaries serving under the Romans. In an assault by Civilis at "Colonia Claudia Ara Agrippinensis" (at modern Cologne), a cohort of Chauci and Frisii had been trapped and burned.
Final demise of the ancient Frisii.
The emperor Constantius Chlorus campaigned successfully against several Germanic peoples during the internecine civil wars that brought him to sole power over the Roman Empire. Among them were the Frisii and Chamavi, who were described in the "Panegyrici Latini" (Manuscript VIII) as being forced to resettle within Roman territory as "laeti" (i.e., Roman-era serfs) in c. 296. This is the last reference to the ancient Frisii in the historical record. However, they appear once more, now in the archaeological record. The discovery of a type of pottery unique to 4th century Frisia known as "Tritzum earthenware" shows that an unknown number of them were resettled in Flanders and Kent under the aforementioned Roman coercion. 
If there were any Frisii left in Frisia, they fell victim to the whims of nature, civil strife and piracy. After several hundred years of favorable conditions, the natural environment in the low-lying coastal regions of northwestern Europe began to deteriorate c. 250 AD and gradually worsened over the next 200 years. Rising sea levels and storm surges combined to flood some areas. Many deserted village sites were silted over. The situation was probably aggravated by a shift to a cooler, wetter climate in the region as well as by the introduction of malaria and other epidemic diseases.
In the 3rd and 4th centuries the population of Frisia steadily decreased, and by the 5th century it dropped dramatically. Archaeological surveys indicate that only small pockets of the original population stayed behind (e.g. in the Groningen coastal marshes).
The coastal lands remained largely unpopulated for the next one or two centuries. As soon as conditions improved, Frisia received an influx of new settlers, mostly from regions later characterized as Saxon, and these would eventually be referred to as 'Frisians', though they were not necessarily descended from the ancient Frisii. It is these 'new Frisians' who are largely the ancestors of the medieval and modern Frisians. Their Old Frisian language, however, was more intricately related to Old English spoken by their relatives settling abroad, than to the Old Saxon language spoken by the people staying behind in Germany.
Arguing against the replacement theory, recent excavations in the coastal dunes of Kennemerland show clear indication of a permanent habitation.
Suggested Roman references.
Auxiliaries at Hadrian's Wall.
One of the entries of the "Notitia Dignitatum" reads "Tribunus cohortis primae Frixagorum Vindobala", referring to the office of a tribune of the first cohort of the 'Frixagi', once stationed at Vindobala (at modern Rudchester) on Hadrian's Wall. Efforts have sometimes been made to connect this auxiliary unit with the Frisii by supposing that the original document must have said "Frisiavonum" and a later copyist mistakenly wrote "Frixagorum". Some works make the claim in passing, perhaps citing someone else's claim of a copyist's error as justification.
The Frisiavones.
The Frisiavones (or Frisiabones) are mentioned in Pliny the Elder's "Natural History" (AD 79). They are listed as a people of the islands in and near the Rhine River, as are the Frisii. They also appear as a people of northern Gaul in the chapter on Gallia Belgica, their name given between those of the Sunici and Betasi (not to be confused with the Batavi).
Tangible evidence of the existence of the Frisavones includes several inscriptions found in Britain, from Roman Manchester and from Melandra Castle near modern Glossop in Derbyshire. The Melandra Castle inscription reads "CHO. T. FRISIAVO C. VAL VITALIS", which may be expanded to become ""Cohortis Primae Frisiauonum Centurio Valerius Vitalis", which may be translated as "Valerius Vitalis, Centurion of the First Cohort of the Frisiavones"".
Suggestions that the Frisiavones were actually the Frisii center on the similarity in names, combined with the Roman classification of 'Lesser Frisii' to the west of the Zuiderzee and 'Greater Frisii' to the east of it (which provides a reason as to why the Frisii might have been known by two different names). However, Pliny's placement of the Frisiavones in northern Gaul is not near the known location of the Frisii, which is acceptable if the Frisavones are a separate people, but not if they are a part of a greater Frisian tribe.
Theodor Mommsen ("The Provinces of the Roman Empire from Caesar to Diocletian", 1885) believed that the Germanic tribes of the region consisted of two parts, one having come under Roman influence and the other having remained outside of Roman influence, and he concluded that the Frisiavones were the same people as the Frisii. However, his reasoning parsed the accounts of Tacitus and Pliny selectively: he interpreted the 'Lesser Frisii' and 'Greater Frisii' of Tacitus to refer to the Roman-influenced Frisavones and the non-Roman-influenced Frisii; he considered Pliny's account that mentioned both the Frisiavones and the Frisii to be consistent with the model; and he rejected Pliny's account placing the Frisiavones in northern Gaul, saying that it "is beyond doubt incorrect".
Early medieval 'Frisian' references.
The "Panegyrici Latini" in c. 297 is the last mention of the Frisii by that name. There is no mention of them by any other name for nearly three centuries, when the name re-emerges as 'Frisians'. These later references are all connected to the