Helena Petrovna Blavatsky (1831-1891), known as Madame Blavatsky, was an influential Russian occultist, theosophist, and writer, who co-founded the Theosophical Society in 1875 with Henry Steel Olcott. She popularized Western esotericism and ancient wisdom, and is best known for her works Isis Unveiled (1877) and The Secret Doctrine (1888).