Most readers will be quite comfortable on how to proceed when their partner opens 1NT. He shows 15-17 balanced. You add your points and know right away if you are in range for a partscore, game invitation, game, slam invitation, or slam. You can use Transfers or Stayman.
But, it is not as comfortable to handle a 1NT rebid. After you read this article, it should be! In fact, when partner rebids 1NT, you actually know more about his hand than had he opened 1NT.
...