Genesis 6:4

There were giants [Hebrew: Nephilim, might or fallen ones] in the earth in those days; and also after that, when the sons of God came in unto the  daughters of men, and they bare [children] to them, the same [became] mighty men which [were] of old, men of renown.


Note:  Nephilim refer's to the fallen angels (see http://www.jewishencyclopedia.com/articles/11442-nephilim)

Genesis 6:2

That the sons of God saw the daughters of men that they [were] fair; and they took them wives of all which they chose.


See Book of Enoch; Books of Josephus; etc for identification of "sons of God"