If the command given by Jesus is to make disciples of all nations (or ethnic groups), then common sense would tell us our job is to find those nations (ethnic groups) that have not been discipled (taught to be followers of Christ). People desiring to fulfill the Great Commission need to know where these "unreached" groups of people are, so that our efforts in completing the task will not just be busy but productive.
WHY THE 10/40 WINDOW?
The vast majority of these unreached people live in an area of the world nicknamed the "10/40 window." The 10/40 window is simply a term used to describe a region of the world within 10 and 40 degrees latitude from Western Africa to Eastern Asia. If you were to draw it on a map, the top would go from Portugal through Japan and the bottom would go from Guinea through the bottom tip of India all the way to the Philippines. This is an important region to think about as a World Christian because most of the people who have not had an opportunity to hear the gospel live here. Joshua Project  states, "The original 10/40 Window included only countries with at least 50% of their land mass within 10 and 40 degrees north latitude. The revised 10/40 Window removes several Christianized countries such as South Korea and the Philippines and includes several additional countries, such as Indonesia, that are close to 10 or 40 degrees north latitude and have high concentrations of unreached peoples." This revised list contains over 65 least-reached countries in this "window."
The world is not just saved and unsaved, but reached and unreached.
The people who are lost in the 10/40 Window are not "more lost" than your neighbor or family member who does not know Christ. But, they are "unreached" in the sense that they have not had an opportunity to hear the Gospel. The issue is not their lostness, but their access to the Gospel. People can be unevangelized without being unreached. There are people in the United States that have not heard the gospel, but they could if they wanted to. Most people living in the 10/40 window couldn't find out about Jesus even if they wanted to! These are unreached people who do not have access to the gospel.
There are over 3.6 billion unreached people in the world today. Of those 3.6 billion people, 88% live in or near the 10/40 window. Only 2.17% of these unreached people live in North and South America combined! "As far as unreached people groups, an estimated 2.91 billion individuals live in approximately 5,626 unreached people groups are in the 10/40 Window. Also, the top 50 least evangelized megacities are all in the 10/40 Window! That fact alone underscores the need for prioritizing 10/40 Window Great Commission efforts."
This area of the world is so unreached for several reasons. First, these people do not live in a spiritual vacuum. The world's major religions began in this part of the world and are firmly entrenched there. In the 10/40 window there are 724 million Muslims, 787 million Hindus, and 240 million Buddhists. Along with that, many of the countries in this region are oppressive to Christianity. Regardless of these facts, Jesus declared that, "the harvest is plentiful but the workers are few." The biggest reason this part of the world is so unreached is because there is a lack of Christians willing to go to these places.
It is estimated that only 3.3% of foreign missionaries today are working to reach these unreached people. The other 96% are working in unevangelized, but not unreached areas. According to the World Christian Encyclopedia of all the money designated for "missions" in the U.S. only 5.4% is used for foreign missions. Of that 5.4%, only 1% is used to take the gospel to unreached people who don't have access to the gospel. That's about one cent out of every one-hundred dollars given to missions! The rest goes towards efforts to further evangelize reached people.
Martin Luther King Jr. said, "Nothing in the world is more dangerous than sincere ignorance," and that proves true in our global strategies for advancing God's kingdom. We must take the time to educate ourselves on what the world looks like and evaluate our efforts in line with God's command to make disciples of all people groups.
 For more great information about the 10/40 Window or Unreached People groups, visit our friends at Joshua Project and find the most up to date information on the state of world evangelization.