Most scientists learn how to review papers by being thrown into the deep end of the pool: here's a paper, write a review. Perhaps this is not the best way to keep an enterprise afloat. But what should be included in a review? What should the tone of a review be? Here I want to outline the specifics of what we at the JCI are looking for in a referee report.For the editorial process to be successful, a few things are paramount: a well-written and novel manuscript, an informed and unbiased editor, and constructive comments from peer reviewers. We've already published pieces on how to write a scientific masterpiece (1) and what to expect from the editors (2, 3), but so far, we haven't paid as much attention to another important group: referees. In addition to the myriad tips listed by the Editor in Chief (4), here are a few other things to keep in mind.