The first three gospels in the New Testament canon are called the Synoptic gospels because they have a "similar perspective" on the life of Jesus (syn- is a Greek prefix meaning "together", and optic comes from a word for "seeing"). If one were to line Matthew, Mark, and Luke up in vertical columns, one would notice that the plotline of the three narratives is quite similar, and that many of the pericopes -- miracle stories, descriptions of Jesus' teaching, and so on -- occur in the same place and use the same wording in each gospel. This is not true of the Fourth Gospel, John, who presents a very different chronology of Jesus' life and a very different perspective of his personality and his method of teaching.

The fact that the stories in the synoptics so often match one another word-for-word suggests that they were all based on written rather than oral sources. The Greek text of the New Testament often betrays an approach reminiscent of modern cut and paste composition. For instance, the editorial aside "Let the reader understand" in Mark 13:14 is repeated in Matthew 24:15, which hardly seems like the sort of thing that would arise naturally from a spontaneous telling of a story (to say nothing of its reference to a reader as opposed to a listener!). Given the fact that Jesus' mother tongue was almost certainly Aramaic, it seems awfully suspicious that renderings of his stories into Greek contain the vast amount of verbatim agreement that can be seen in the Synoptics.

So: How are the synoptics related? Who copied from whom? Were there once written sources about Jesus' life which are now lost to us? What is the relationship between these written sources and the oral stories that circulated during and after Jesus' lifetime? This constellation of questions formulates the Synoptic Problem.

For the historians who address this issue, the subtle differences between the synoptic gospels become crucially important. All three evangelists have some material that is unique to them. For instance, the story of Jesus healing a blind man with his spittle in Mark 8 appears nowhere else; the assurance that Jesus came not to abolish the law but to fulfil it appears only in Matthew 5; and the visit to Herod near the end of Jesus' life appears only in Luke 23. But even in the stories that the synoptics have in common, tiny differences can sometimes be found which betray each evangelist's political and religious agendas. To choose just one example, in Mark's version of the parable of the sower (4:3-20), Jesus loses patience with his disciples and snaps at them for being dense. (This is a common theme in Mark.) In both Matthew's version (13:3-23) and Luke's version (8:5-15), Jesus appears gentler, patiently providing an explanation for the parable without expressing any annoyance. From moments like these, we can learn a little about what motivated each writer.

Even in the earliest years of Christianity, it was observed that Mark is by far the shortest of the synoptics, his vocabulary the crudest, his Christology the lowest, and his story the simplest. Modern scholars take this to mean that his gospel was written first, and that it was later expanded (and somewhat cleaned up) by Matthew and Luke. This is called the theory of Markan priority and it is held by the vast majority of historians of religion today. In the Middle Ages, it was more common to believe that Matthew came first and that Mark edited and condensed his gospel. But for various reasons this view is no longer held, except by some evangelical Christians who have an emotional investment in Matthean priority. (Matthew was a disciple; Mark was not. This makes Markan priority an embarrassment for certain literalist Christians, though the point is moot for most serious New Testament scholars since they don't believe that the gospels were written by eyewitnesses anyway.)

Matthew and Luke, for their part, have a lot of material in common which does not appear in Mark. The material that Matthew and Luke share, which Biblical scholars call the double tradition, has provided something of a puzzle. Where did it come from? This is a particularly thorny question since it seems that Matthew and Luke worked independently of one another. The dramatic differences between their birth narratives and their crucifixion stories suggest that they were not reading each other's work; furthermore, there are a number of points where Matthew tells a story that closely fits Luke's agenda and which Luke nevertheless does not use. As a result, it seems likely that the two of them were reading another document, now lost, which scholars call Q. (It was German scholars who first developed this theory, and the German word for "source" is Quelle. The shorthand name stuck among speakers of other languages, including English.) The double tradition contains some of the most memorable teaching of Jesus, including the Lord's Prayer and the Sermon on the Mount. It is therefore important to figure out where this material came from, and whether it can be traced back to the Historical Jesus.

The view that I just described, which is to say that the view that Matthew and Luke drew on Mark and Q when compiling their gospels, is called the Two-Source Hypothesis. Today most scholars of the New Testament take the existence of Q for granted, and some, most notably John Kloppenborg and Burton Mack, have gone through the trouble of tracing Q's history and analyzing its strata. (Tricky business, given the fact that Q is completely hypothetical!)

There are a few dissenting voices, however. Mark Goodacre, who teaches at the University of Birmingham, is a vocal proponent of a version of the Farrer hypothesis, which posits that Matthew and Luke were not independent after all. Goodacre accepts Markan priority, but he rejects the existence of Q. He believes that Luke based his gospel on Mark and Matthew, and that the signs that other scholars interpret as proof of Matthew's and Luke's independence can be explained in other ways.

Other solutions to the Synoptic Problem have been proposed, some of which include other, non-Q hypothetical documents like proto-Mark, ur-Mark, proto-Matthew, and so on. However, these theories generally lack the explanatory power necessary to put difficult questions about the composition of the gospels to rest.