A Checklist for antitrust attorneys to use when counseling parties to a horizontal merger with potential anticompetitive effects in the US. This Checklist discusses what to do before making a Hart-Scott-Rodino (HSR) Act filing, how to advise clients on the preparation of competitive material, how to ensure that the merging parties do not engage in gun-jumping, how to assess the deal's competitive risks, and how to negotiate antitrust risk-shifting provisions. It also discusses how to manage the HSR process and the response to a Request for Additional Information and Documentary Material, known as a Second Request.