Ask HN: How do you run code reviews?

I am hoping some of you would share a bit of practical experience with code reviews, in particular I would love to get some answers to the following questions: Do you use any tools? How many people review a particular piece of code? Is it a 1-to-1 process? Any rules for the communication associated with the review? I have heard rumors that to avoid the review turning into a discussion, some companies does not allow the reviewee to answer review comments. Does the amount of code review change with the progress of the project or the seniority of the developer?