It is considered to cover roughly 400 years, spanning from the ministry of Malachi (c. 420 BC) to the appearance of John the Baptist in the early 1st century AD.
It is roughly contiguous with the Second Temple period (516 BC–70 AD) and encompasses the age of Hellenistic Judaism.
It is known by some Protestants as the "400 Silent Years" because it was a period when no new prophets were raised and God revealed nothing new to the Jewish people.
[1] Many of the deuterocanonical books, accepted as scripture by the Catholic Church and Eastern Orthodoxy, were written during this time, as were many pseudepigraphal works, the Biblical apocrypha and Jewish apocrypha.
An understanding of the events of the intertestamental period provides historical and literary context for the New Testament.