The apostle John, brother of James and son of Zebedee, one of Jesus' original twelve disciples/apostles, is traditionally viewed as the author of the Gospel of John. According to church tradition he was the only one of the Twelve to live to an old age and die a natural death. This would mean either that he refers to himself in the Gospel, or that the final author/editor/compiler refers to him, as the "disciple whom Jesus loved". This disciple is described as being present at Jesus' crucifixion, however, which is difficult to fit together with the information from the Synoptic Gospels, which suggest that all of the Twelve fled when Jesus was arrested, and only Peter is said to have followed at some distance (see further Mark 14:50, 54, 66-72). However, it is clear from John's Gospel, that John himself was meticulous in his description of the events surrounding the Crucifixion, including Jesus placing his mother Mary in the care of John.
There is at any rate significant debate about whether the Gospel of John was actually written by John the apostle. From internal evidence, one might conclude that Lazarus was the author, since he is the only named individual specifically referred to as the one whom Jesus loves (John 11:3). However, this reference to Lazarus is only a secondary reference to one of many people who Jesus loved and cannot easily support authorship of a Gospel.
Although authorship is still debated, historical testimonies strongly support John, the son of Zebedee, as the author of this Gospel as well as the other Johannine works (Epistles of John, Revelation). External evidence is found in the writings of the church fathers -- Ignatius, Polycarp, Tatian, Theophilus -- who testify that John wrote these works. Eusebius also testifies to the fact that John wrote his Gospel last of all, between 80 - 95 AD. This is further substantiated by Irenaeus who wrote that John eventually moved to Ephesus and lived to be an old man, living during the reign of Trajan (98 - 117 AD).