“The Dead Sea Scrolls are undoubtedly the most important discovery found in Israel in the field of the Bible and history of Judaism and Christianity.”1 Fragments of about 900 scrolls from the 2nd Temple period (some dating as early as 3rd century BC) were found in the Qumran caves (NW shore of Dead Sea), between 1947 and 1956. Every book of the Old Testament (except Esther) were represented in the finds, including one complete copy of Isaiah.

Some of the Qumran Caves where the Dead Sea Scrolls were discovered (1946ff.). Photo by Leon Mauldin.
I remember when studying archaeology under the late Dr. James Hodges that he said the main value of the Dead Sea Scrolls was in the discipline of apologetics. The scrolls were about 1,000 years older (!) than the previously available manuscripts with which translators had to work. The huge find of fragments provided abundant samples with which to compare our Hebrew manuscripts. The result was:
it may now be more confidently asserted than ever before that the modern Hebrew text faithfully represents the Hebrew text as originally written by the authors of the Old Testament. Dead Sea discoveries have enabled us to answer this question with much greater assurance than was possible before 1948.2
Dr. Hodges pointed out that no new translations had to be made as a result of the discovery of the scrolls; they confirmed the accuracy of transmission of what we already have.
Another contribution:
“As a result of Dead Sea Scroll discoveries, it is no longer possible to date portions or entire Old Testament books as late as some scholars used to do. It is impossible to date any biblical work or any extensive part of one later than the early second century B.C. Fragments of the Pentateuch and the prophets date from the second century B.C. Ecclesiastes, sometimes believed to have been composed in the second or first century B.C., appears in a Cave 4 manuscript dating from 175 to 150 B.C. A second-century B.C. Copy of the Psalms indicates that the collection of Psalms was fixed by Maccabean times. A manuscript of Daniel dating about 120 B.C. brings into question the alleged Maccabean date of its composition. Moreover, the Dead Sea Scrolls do not support the existence of a deutero- or trito-Isaiah, at least during the second century B.C. The complete Isaiah and the long fragment of Isaiah from Cave 1 (second century B.C.) treat the book as a unit.”3
Click photos for larger view.
1 Yigael Yadin quoted by Hanan Eshel in Qumran: Scrolls∙Caves∙History (p.7)
2 See F. B. Bruce, Second Thoughts on the Dead Sea Scrolls (pp.61-69)
3 Bible and Spade (1978), 7(1), pp.12–14.