The ancient history of Turkey testified massive migrations, with intervals of a thousand years each. The first of these migrations occurred in last centuries of the Third millennium BC and the second migration towards the last two centuries of the second millennium BC. After about a thousand years from the latter, another massive migration, which composed the topic of my paper, spread to the areas of modern Balkan States. There are accounts relating to this migration in ancient literary sources written in Greek and Latin languages. A discussion referring to the ethnicities of these immigrants, will not be made because they were the first indigenous people and they are mentioned as Celts (Keltoi) and Gauls (Galli) by ancient Greeks and Romans respectively. A groups of them that passed to Anatolia, were especially mentioned as the Galatians in ancient literary sources. They settled northern Phrygia including the upper region of Halys (modern Kızılırmak), and later this area was named Galatia after the Galatians. Nevertheless accounts in ancient literary sources are inadequate or inconsistent with their deeds after passing to Anatolia, where they came from? Why and how they did? Which paths they used? The aim in this paper is to research the reason, developments and paths of the Celtic/Galatian migration, and their deeds after the migration by using literary sources and epigraphic, numismatic and modern studies.