The angel. This was probably Gabriel, who had already appeared to Zacharias (Luke 1:11, 19); and to Mary (see on Luke 1:19).
In a dream. Luke (ch. 1:26-38) implies that the angel appeared to Mary visibly, not in a dream or a vision: he “came in unto her” (v. 28). To Joseph, however, the angel appeared in a dream while he was brooding over the problem that perplexed him.
Inspired dreams are one of God’s appointed ways of revealing His will to men (Num. 12:6; Joel 2:28; cf. Gen. 20:3; 31:11, 24; 41:1; etc.)
Thou son of David. Joseph knew, of course, that he was of the royal line. He may even have been heir apparent to the throne of David, as could be implied by Matthew’s genealogy.
Fear not. He was not to hesitate or to question Mary’s virtue. As a “just man” (v. 19) Joseph need have no fear that by taking Mary he would forfeit righteousness. In fact, God required this act of faith.
Wife. Gr. gune, meaning (1) woman in general (chs. 9:20; 13:33; etc.), (2) a wife (chs. 14:3; 18:25), (3) one betrothed (Gen. 29:21, LXX; Deut. 22:23, 24, LXX; cf. Rev. 21:9). Here the third meaning evidently applies.