There have been three pronounced periods in the historical eras of modern missions. William Carey is frequently attributed as the father of modern missions.
In the mid-19th century, God again uses the passage from Matthew 28:18-20 to spur Hudson Taylor to shift the focus from the coastlands to the inlands. Hudson Taylor had already surrendered to the call to go to China, but while in Shanghai, he soon realized that many missionaries were staying in coastal cities. In contrast, most places further inland remained untouched by the gospel.
As missionaries traveled inland, they encountered myriads of different cultures, languages, and religions. Influenced by Dr. Donald McGavran, Dr. Ralph Winter took the HUP and, in the 1970s, began to reshape strategy and thought of modern missions. Winter speaking at the 1974 Lausanne Congress on World Evangelization, presented an argument informed by the HUP that “reignited cross-cultural evangelism while restoring to many. The third wave of modern missions has focused on prioritizing unreached people groups.
It's too early to tell, but perhaps diaspora missions is the fourth wave of modern missions. The global diaspora has significant implications for those that consider the Great Commission a command to be lived out by all followers of Jesus of all ages. We now see that God is bringing people from difficult access and gospel-deprived regions to sociopolitical nations with substantial evangelical populations with governmental protections against religious persecution and freedom of religion. We currently see Europe and North America as strategic mission fields for the unreached.