Genesis 9:12 - 15

God said, �This is the sign of the covenant that I make between me and you and every living creature that is with you, for all generations: I have set my bow in the clouds, and it shall be a sign of the covenant between me and the earth. When I bring clouds over the earth and the bow is seen in the clouds, I will remember my covenant that is between me and you and every living creature of all flesh; and the  waters shall never again become a flood to destroy all flesh�

God made a solemn agreement with Noah that never again would He cause a flood to destroy all living creatures. The rainbow was to be a perpetual sign of this promise.