Modern federal appellate review of sentences is a recent phenomenon introduced by the Sentencing Reform Act of 1984. Before United States v. Booker, courts of appeal focused on enforcing the technical rules of the federal sentencing guidelines and did so with (over)zealous enthusiasm. In the new post-Booker world, appellate judges are supposed to review sentences for "reasonableness." But how are they supposed to determine what is - or is not - a reasonable sentence? The answer to this puzzle rests in the mind of the District Judge. This short essay argues that the sentencing judge must explain his reasons, and meaningfully document - in the form of sentencing opinions - how he grappled with the relevant statutory factors from the Sentencing Reform Act of 1984 in reaching the sentence imposed.