A preliminary injunction is the most powerful thing a court can do before it knows who is right, and courts guard it accordingly. This article explains what the four factors actually require after Winter v. NRDC replaced the possibility of irreparable harm with a likelihood of it, why irreparable harm is where most motions fail, and how the surviving sliding-scale formulations differ across circuits. It distinguishes prohibitory from mandatory relief and explains why asking a court to change the status quo triggers a heightened standard that many movants never see coming. It walks the procedural machinery of Rule 65 in the order it matters: the ex parte temporary restraining order and its fourteen-day life, the notice and hearing requirements, the security bond and what happens when a wrongfully enjoined party collects on it, the specificity and binding-persons requirements of Rule 65(d) that make an order enforceable, and contempt including the collateral bar rule that requires obedience even to an erroneous order. It closes with the statutory bars that forbid injunctions outright, the interlocutory appeal, and the unsettled state of relief that reaches beyond the parties.