The Two Witnesses are two human beings who will be given extraordinary power for 3 1/2 years just before Christ's return (Revelation 11:3). If anyone attempts to harm them before they complete their ministry, "fire proceeds from their mouth and devours their enemies" (verse 5). Note the similarity between this and the power given to Elijah (II Kings 1:9-15). They will also have power to produce drought and famine, as did Elijah (Luke 4:25; I Kings 17:1-7), and to smite the earth with plagues, as Moses did in Egypt (Revelation 11:6). However, there is no biblical justification to believe that the Two Witnesses are indeed Moses and Elijah; God has never resurrected a prophet or apostle to do a work centuries or millennia later. God always uses people of the time to carry out His work.
The most notable work of the Two Witnesses will be to give one final warning to this world. This message of rebuke and repentance will bring them into conflict with the people and the governments in power, and they will be martyred. The whole world will rejoice and give each other gifts as they view their dead bodies, lying unburied in the streets of Jerusalem (verses 9-10). However, those same people will be struck with great fear when they see the Two Witnesses come to life again and ascend into the sky in a cloud (verses 11-12).
Verse 4 calls them "the two olive trees and the two lampstands standing before the God of the earth," an obvious reference to Zechariah 4:14. Combining this Old Testament prophecy with the interpretation of the lampstands in Revelation 1:20, it appears that the Two Witnesses will also have a profound ministry to the church of God: They will supply the church with "oil," that is, they will spiritually feed the church during the "famine of the Word" (Amos 8:11). Thus, the Two Witnesses will be affiliated with and teaching the members of God's church before and possibly during their greater worldwide ministry.