There's a fairly comprehensive introduction to western Sufism available here pps 7-21. Fundamental to the story — the progenitors of one of its two principle streams — is the influence of the Traditionalists, notably René Guénon and Frithjof Schuon. The other stream is that of Pir-OMurshid Hazrat Inayat Khan (a Moslem born and raised in India), who's Sufi Order introduced a fundamental innovation: he separated Islam from Sufism, presenting it rather as a universal doctrine independent of the exoteric religious traditions. On a personal note, being aware of Guénon and Schuon, they represented and spoke for the Sophia Perennis, an expression of the Primordial Tradition founded almost single-handedly by Guénon, who at the time was under the influence of the Hindu Tradition and who regarded Hindu metaphysics as the only true and unadulterated expression of a universal metaphysics. Both became Sufis, but neither really ever presented their Sufi faith as a mainline teaching, rather treating Islam as one among a number of Divine Revelations. It's a matter of note that, when a Sufi living in Cairo, it took two other Trads, the Sufi Martin Lings and the Tibetan Buddhist Marco Pallis, to convince Guénon that Buddhism was an authentic Tradition in its own right, and not a 'Hindu heresy' as he had been taught by his gurus. What can be seen is that Khan's universal Sufism soon became intermixed with various other eastern mysticisms and self-declared avatars, and was led by those who sought initiation from all and sundry they met on their path, flouting the age-old rule of trying to ride two or more horses at the same time. The Traditionalist movement has really been in something of a decline with the passing of those great names allied to it, and a general drifting away from the enthusiasms of the New Age and the quest for 'spiritual enlightenment' (largely today a quest for personal wellbeing and contentment). What remains is heavily influenced by western eclectic sufism, and the spokespersons, nearly all Sufi I believe, take quite a high-handed view towards other traditions, notably Christianity, which they insist has 'lost the plot' — they make the fundamental error of failing to discern between eso- and exoteric, assuming that a coarsened or distorted exoterism, such as 'fundamentalism' in the US, defines the whole tradition, which is tantamount to suggesting that terrorists and suicide bombers defines Islam. Lastly I would say, in response to Phyllis Sidhe Uaine, post: Specifically, what are the similarities and the differences between Sunni and Sufi Muslims? I would say that 'western Sufism' bears a scant and very distant resemblance to the Sufism recognised by Muslims, be they Sunni or Shia, or Sufism as an esoteric component of Islam.