A Practice Note discussing the issues counsel should consider when advising merging parties to a corporate transaction with effects in the US. This Note discusses compliance with merger control procedures under the Hart-Scott-Rodino Antitrust Improvements Act of 1976 (HSR Act) and other antitrust laws, how to conduct a preliminary substantive assessment of a transaction's competitive effects even if not HSR-reportable and even after the transaction closes (post-consummation review), and provides an overview of the HSR review process, including how to comply with a Request for Additional Information and Documentary Material, known as a Second Request.