What is Pentecostalism? How can it best be understood? It is best described as a series of renewal movements that stem from the early days of the church. Modern Pentecostalism dates from the start of the 20th century. The Introduction looks at the roots of modern Pentecostalism and tries to explain its diversity around the world. How has Pentecostalism managed so successfully to survive and indeed thrive in recent decades despite the general wane in Christian religious following of the last century?