The Farrer theory (also called the Farrer-Goulder hypothesis) is a possible solution to the synoptic problem. The theory is that the Gospel of Mark was written first, followed by the Gospel of Matthew and then by the Gospel of Luke.
It has mainly been advocated by English biblical scholars. It is named for Austin Farrer, who wrote On Dispensing With Q in 1955, but it has been picked up by other scholars including Michael Goulder and Mark Goodacre.
The Farrer theory has the advantage of simplicity, as there is no need for hypothetical sources to be created by academics. Instead, advocates of the Farrer theory argue, the Gospel of Mark was used as source material by the author of Matthew. Lastly, Luke used both of the previous gospels as sources for his Gospel.