Antitrust law is the body of rules that decides when cooperation between businesses becomes conspiracy and when winning becomes monopolizing. This article explains the three foundational statutes, the difference between conduct that is condemned automatically and conduct judged by its net effect, and why market definition quietly decides most cases. It walks through horizontal agreements, vertical restraints, monopolization, merger review, and price discrimination using examples drawn from ordinary businesses rather than famous corporations. It explains who may sue, what treble damages actually mean, and why the person who pays the overcharge is not always the person allowed to recover it. It also covers the antitrust exposure that small and midsize companies genuinely face, including trade association meetings, no-poach clauses, and casual pricing conversations. Readers finish knowing which behaviors carry criminal risk, which are merely risky, and what to do when a subpoena arrives.