Merger control is the one area of antitrust where the government gets to see the crime before it happens, and this UpLaw guide maps the whole system from the statutory standard to the courthouse steps. It explains Clayton Act Section 7 and its deliberately loose "may substantially lessen competition" incipiency test, then walks the Hart-Scott-Rodino premerger notification regime in operating detail: who the filing persons are, how the size-of-transaction and size-of-person tests work, which exemptions actually matter, how the waiting period runs, and what happens when a Second Request lands. It then turns to substance under the 2023 DOJ/FTC Merger Guidelines, covering market definition and the hypothetical monopolist test, the HHI structural presumption, unilateral and coordinated effects, potential competition, entry, efficiencies, the failing-firm defense, vertical foreclosure, and serial-acquisition theories. Remedies, the preliminary-injunction posture and the Baker Hughes burden-shifting framework, gun-jumping and interim-conduct limits, antitrust risk allocation in the merger agreement, and multi-jurisdictional filings round out the treatment. A running hypothetical follows an industrial-valve deal from term sheet through Second Request to a negotiated divestiture, and inline cross-references route you to UpLaw's deeper treatments of antitrust doctrine, HSR filing mechanics, M&A process, and compliance.