A biblical generation is defined as a forty year time period. If we look at recent American history, it is easy to see where these generations are delineated.
Around 1908, the high point of the Azusa Street Revival, God began to pour out his Spirit in many places in the world. The Pentecostal movement had its beginning in this revival and the Great Welsh Revival had begun a few years earlier. From Eastern Europe to India, Christians shared a common experience of refreshing and a renewal of the gifts of the Holy Spirit being restored to full operation in the Church.
Forty years later, in 1948, the same year that Israel became a nation, Billy Graham began his ministry. The Latter Rain movement, Healing Revival, and Charismatic movement had their roots in events that took place during this year. The apostolic, prophetic and evangelistic ministries were restored in a greater measure for the purpose of equipping Christians for works of service.
Then in 1988, the Spirit was poured out like never before. We have yet to see the full extent of this revival, but it is probable that it will bring global awakening, the dissolution of tyrannical governments and the restructuring of man-made hierarchies in the church. Much of what happened in the news in 1988 and 1989 was miraculous and can only be explained by a sovereign outpouring of the Spirit of God with power.