The Picts did not leave us any written sources. It is possible that there once were Pictish manuscripts but they did not survive. Luckily, a few contemporaries of the Picts mention them in their works. There are Roman, Latin, sources in which the Picts are mentioned and there are Insular sources in which the Picts appear.
In a lot of publications about the Picts, scholars still claim that Eumenius from Autun (230/240 – around 312) is the first one who mentioned the Picts. However, the panegyric (a formal public speech in which a person or thing is highly praised) that first mentioned the existence of the Picts is no longer attributed to Eumenius. This panegyric was written by an anonymous author in 297 to praise the emperor Constantius Chlorus (c.250 – 306). In this praise speech the author compares Constantius’ achievements in Britain to the easier task that faced Julius Ceasar in dealing with the Britons. The ‘Picti’, as he called them, and the Hibernians (Hibernia is the classical Latin name for Ireland) were savages and a far more difficult opponent compared to the Britons. This panegyric shows us that the Romans were aware of the existence of the Picts at the end of the third century. The anonymous author used them in his praise speech because he knew that the Roman public had knowledge of the Picts, and that they were aware of the fact that the they lived in the northern part of Britain.
There is another panegyric that mentions the Picts. This panegyric was also written by an anonymous author and also praises Constantius Chlorus. The praise speech, that was written in 310, is transmitted through time in a manuscript that is known as the Panegyrici Latini. This manuscript is a collection of twelve ancient Roman and late antique panegyrics. The other panegyric that mentions the Picts, the one from 297, is also part of this manuscript. The original Panegyrici Latini was discovered in 1433. This original manuscript has perished but there are a few copies that did survive. One of those copies is Ms. Harley 2480, which is kept at the British Library and was written in the second quarter of the fifteenth century.
Ammianus Marcellinus (c. 330 – 400) was an historian. He has written the Res Gestae, a history of Rome from the accession of the Emperor Nerva in 96 to the death of Valens at the Battle of Adrianople in 378. In this history he briefly mentions the Picts who, according to him, harassed the Britons with raids around the year 360. He claims that the Picts were divided into two tribes, the Dicalydones and the Verturiones. The eldest surviving copy of the Res Gestae dates from the ninth century and is kept at the Biblioteca Apostolica Vaticana at the Vatican, the manuscript is known as Ms. Vat.Lat. 1873.
Claudius Claudianus (c.370 – c. 404) was a Roman poet. A lot of his work has survived through time. He mentions the Picts a few times in his works, portraying them as savages that tattooed or painted their bodies. It is not certain if the Picts actually had tattoos or painted bodies because Classical writers often ascribed this trait to barbaric people. It was a way to let the reader know that they were dealing with barbarians. Isidore of Seville (560 – 636) and Jordanes (sixth century) repeated this claim. They also mentioned the tattooed Picts in their works but they based their claims on Claudius. The Insular sources, writers that lived in Britain and probably have met the Picts in real life, have never mentioned painted or tattooed bodies of the Picts.
Gildas (c. 500 – c. 570) was a monk who most probably was born in Scotland. He has written De Excidio et Conquesto Britanniae about the history of the Britons before and during the coming of the Saxons. He wrote his polemic at the monastery that he founded in Rhuys in Brittany (Northern France), now known as St. Gildas de Rhuys. His work (loosely translated it means: On the ruins of Britain) focuses on the withdrawal of the Roman protection, leaving Britain open to savage peoples, including the Picts. Gildas condemns his contemporaries who did nothing, or too little, to prevent the Saxons from entering Britain and who let other savage people, including the Picts and the Scots, ravage their lands. Gildas was not the only one who thought of the Picts as a vicious people. Saint Patrick (end of the fourth century – 461), known as the Apostle of Ireland, agreed with him. According to Saint Patrick, the Picts had bought converted Irish slaves. He condemned this action, calling them ‘the most shameful, wicked and apostate Picts.’
The most important source about the Picts is the Historia Ecclesiastica Gentis Anglorum from the monk author, the Venerable Bede (672/673 – 735). Bede provides an extensive history of Britain in which he mentions the Picts multiple times. He lived at the monastery of Monkwearmouth-Jarrow where he also wrote his Historia Ecclesiastica Gentis Anglorum. His work serves as a starting point for many studies on the Picts, although some scholars question the reliability of some of Bede’s accounts. It is, for example, highly unlikely that the Picts adhered to a matrilineal descent. Bede did not think of the Picts as a savage and vicious people. Just as Adamnan (c. 624 – 704), who wrote the life of Saint Columba (521 – 597), he saw the Picts as a civilized people. In the life of Saint Columba, Adamnan mentions the encounters which Saint Columba had with the Picts. Columba came originally from Ireland and founded a monastery on Iona, an isle on the west coast of Scotland. He claims that even the magicians of the Pictish kings appear restrained in their efforts to combat the threats of Christianity.
Another important primary source are the surviving King-Lists. These lists make up a record of the Pictish kings in chronological order. Unfortunately, they do not record the dates when the kings reigned and the multiple surviving lists disagree with each-other in the names of the kings. The King-Lists are complex, and I have not studied them to any extent. For the different King-Lists and a brief introduction to these materials visit this very helpfull website: http://www.historyfiles.co.uk/KingListsBritain/GaelsPictland.htm and for an extensive history of the Picts and the Pictish succession, I would recommend: J.E. Fraser, From Caledonia to Pictland, Scotland to 795, Edinburgh, 2009, and A. Woolf, From Pictland to Alba, 789-1070, Edinburgh, 2007.
 Nixon 1994, 104-106.  Nixon 1994, 104-144.  Ritchie 1992, 3-4.  https://www.bl.uk/catalogues/illuminatedmanuscripts/record.asp?MSID=3713&CollID=8&NStart=2480  http://www.mss.vatlib.it/guii/console?service=present&term=@5Vat.lat.1873_ms&item=1&add=0&search=1&filter=&relation=3&operator=&attribute=3040  Ritchie 1992, 4-6.  Ritchie 1992, 7-8.  Ritchie 1992, 8.