The Deuterocanonical Books
The deuterocanonical books (deutero books for short) are the seven books that Catholics acknowledge in their Bible in addition to the books in your everyday Protestant Bible. Protestants call these books, "apocrypha" because while they acknowledge their wisdom, they fail to realize their inspiration. These books include 1 and 2 Maccabees, Judith, Tobit, Sirach (also "Ecclesiasticus"), Wisdom, and Baruch. The proof behind both sides of this issue is mainly historical. Let's first look at the Protestant view.

Protestant View
Almost all Protestants believe that the Catholic Church, during the reformation, added the deutero books to the Bible. They claim that the Bible had never included the deutero books, and that Catholics were just putting them in the Bible so that they could make references to certain things such as Purgatory which aren't mentioned in the protocanonical books. Many claim that never before the Council of Trent were the deutero books listed as Scripture.

Disproving the Protestant View
The Protestant view is extremely faulty. The deutero books have been acknowledged as canonical Scripture since the beginning of Church history. They were in fact instituted numerous times before the reformation as infallible Scripture. In the beginning, when the Church first started, they had no recognized list of infallible Scripture. For a while, the people of the Church couldn't be perfectly sure what books were inspired and what were not.

The canon of Scripture, Old and New Testament, was finally settled at the Council of Rome in 382, under the authority of Pope Damasus I. It was soon reaffirmed on numerous occasions. The same canon was affirmed at the Council of Hippo in 393 and at the Council of Carthage in 397. In 405 Pope Innocent I reaffirmed the canon in a letter to Bishop Exuperius of Toulouse. Another council at Carthage, this one in the year 419, reaffirmed the canon of its predecessors and asked Pope Boniface to "confirm this canon, for these are the things which we have received from our fathers to be read in church." All of these canons were identical to the modern Catholic Bible, and all of them included the deuterocanonicals.

This exact same canon was implicitly affirmed at the seventh ecumenical council, II Nicaea (787), which approved the results of the 419 Council of Carthage, and explicitly reaffirmed at the ecumenical councils of Florence (1442), Trent (1546), Vatican I (1870), and Vatican II (1965).

As you can see, the deutero books were confirmed as Scripture about seven times before the reformation even started. The Protestant argument is that not all of these were ecumenical, and were therefore fallible. But they commonly leave out the Councils of II Nicea (787) and Florence (1442) which were both ecumenical and infallible.

In conclusion, the whole basis of the Protestant argument is severely flawed, and in a practical sense, worthless.

The Catholic View, the Proof Behind it, and its Misunderstandings
The Christian acceptance of the deuterocanonical books was logical because the deuterocanonicals were also included in the Septuagint, the Greek edition of the Old Testament which the apostles used to evangelize the world. Two thirds of the Old Testament quotations in the New are from the Septuagint. Yet the apostles nowhere told their converts to avoid seven books of it.

The confusion came when Jews met to discuss the canon of their own religion. There had recently been new books thought of by Christians as Scripture-- the Gospels. The Jews, in an attempt to preserve their religion, immediately cut the Gospels and seven other books (the deutero books) from their Scriptural canon. And so a new Septuagint had been formed, the one Protestants use today. Protestants sometimes claim that their Scripture is more ancient than that of Catholics, but it is, in fact, the other way around.

Furthermore, The New Testament makes references to the deutero books. 2 Macc 6:18-7:24 is referred to in Hebrews 11:35. Wisdom 3:5-6 is referred to in I Peter 1:6-7. Wisdom 13:1-9 is referred to in Romans 1:18-32. If God sees it fit to make references to the deutero books, I think that alone is a worthy cause for their validity. The limited amount of references given are few in comparison to the true amount of references there are. For a huge list of where the deutero books are referred to, take a look at the following site:
http://www.cin.org/users/james/files/deutero3.htm

Some people think that Catholic acknowledge the deutero books as wise and good for teaching, but not inspired or not on-par with the rest of the books of the Bible. The origin of this rumor is unknown to me, but rest assured that it is a rumor, and nothing more. Catholics have always considered the entire Old Testament books as Scripture, and all infallible, equal Scripture. The Council of Trent makes this clear:

If anyone does not accept as sacred and canonical the aforesaid books in their entirety and with all their parts, as they have been accustomed to be read in the Catholic Church and as they are contained in the old Latin Vulgate Edition, and knowingly and deliberately rejects the aforesaid traditions, let him be anathema.

As you can see, the Catholic Church regards all the books of the Old Testament, including the deutero books, as part of one, God-inspired, infallible Bible, which is made up as one, with all of its parts.

Protestants Realizing the Truth?
I actually heard one website, which was in opposition to deutero books, say that just because the early Church Fathers used them doesn't mean they are Scripture. The Truth is, the ENTIRE BIBLE was instituted (not written) by the early Church Fathers. Luckily, most Protestants don't use this argument. In fact, many Protestants are beginning to realize the Truth, and are reading or even using the deutero books.

The Protestant patristics scholar J. N. D. Kelly writes: "It should be observed that the Old Testament thus admitted as authoritative in the Church was somewhat bulkier and more comprehensive than the [Protestant Old Testament] . . . It always included, though with varying degrees of recognition, the so-called Apocrypha or deutero-canonical books. The reason for this is that the Old Testament which passed in the first instance into the hands of Christians was . . . the Greek translation known as the Septuagint. . . . most of the Scriptural quotations found in the New Testament are based upon it rather than the Hebrew.. . . In the first two centuries . . . the Church seems to have accept all, or most of, these additional books as inspired and to have treated them without question as Scripture. Quotations from Wisdom, for example, occur in 1 Clement and Barnabas. . . Polycarp cites Tobit, and the Didache [cites] Ecclesiasticus. Irenaeus refers to Wisdom, the History of Susannah, Bel and the Dragon [i.e., the deuterocanonical portions of Daniel], and Baruch. The use made of the Apocrypha by Tertullian, Hippolytus, Cyprian and Clement of Alexandria is too frequent for detailed references to be necessary" (Early Christian Doctrines, 53-54).

The deutero books are now being published separately from the Bible, so that Protestants can read them without betraying their heritage of Luther, who threw these books out along with James, Revelation, and a couple other books.

The Truth Behind the Protestant Actions
From a Catholic perspective, it can be seriously argued that the Protestants threw out the deutero books because they opposed Protestant beliefs. This would explain why in addition to the deutero books, Luther also rejected Hebrews, James, Jude, and Revelation. Is it a coincidence that all four of the latter books are heavy defenses of the Catholic belief of faith+works for salvation? I think not. All four of the books mentioned give Scriptural evidence that salvation is not by faith alone, especially James. I think that the right thing to do would be to believe what the books say, and believe in the Catholic view on justification. Luther, on the other hand, figured he'd just throw the books right out of the Bible.

In addition, it is no wonder that Luther decided to get rid of the deutero books as well. Tobit proves the validity of veneration of the saints. Maccabees give strong evidence for Purgatory. Sirach emphasizes the free will of man and greatly lessens the more strict views on predestination. And the rest play their purpose in Christianity as well. 

In conclusion, the deuterocanonical books are indeed Scripture, and always have been. They were and are issued as Scripture by the Catholic Church, which has taught Truth, unchanged, since the beginning.