After a more extended bibliography and historical survey of Simonian studies than have hitherto appeared, this study attempts to reduce some of the many uncertainties about Simon and Slmonlanism by a systematic examination of primary sources. The Simon of Acts 8 was an historical Samaritan who repented of his pretensions and was not, pace Irenaeus, responsible for the rise of the Simonian movement. The extant text of Justin (attempts to recover the contents of his lost Syntagma are rejected) supports the double Simon hypothesis and fits in well with the preferred reading of Acts 8. Justin shows too that primitive Simonianism was non-Gnostic, being a pagan, religion dating from the mid-first century in which Simon of Gitta was equated with Zeus and Helena his companion with Athene. Simonianism fell, a century later, under the spell of Gnosticism, and the author seeks to explain various of the inconsistencies of Simonian doctrine as caused by an imperfectly successful attempt to reconcile primitive Simonian with Gnostic ideas. The extant text of Hippolytus (the Syntagma account is adjudged to be almost entirely unrecoverable) adds little to our knowledge of Simonianism proper but it gives us a valuable account of a probably unrelated movement, composed largely of heretical Samaritans, whose Bible was the Megale Apophasis. By the time of Epiphanius the Simonians were addicted to gross orgiastic rites and were probably in decline, though some may have survived till the early fifth century. In the final chapter the author gathers together the information gleaned from the analysis of sources. He contends that Simonianism was not an original religion; it was not the first Christian heresy; it was not the earliest form of Gnosticism. It was essentially derivative and parasitic, an intellectually undistinguished farrago of ideas borrowed from pagan classical religion, from Christianity and from Gnosticism.