A security interest does two entirely different things, and confusing them causes most of the losses in this field. Attachment settles rights between the lender and the borrower. Perfection settles rights against everyone who was never part of the deal — competing lenders, buyers, and a bankruptcy trustee who appears as a hypothetical creditor on the petition date. This article explains why the public record that decides priority contains almost none of the information a stranger would want, why the debtor's name on that record is the most dangerous detail in commercial finance, and how the priority rules actually order competing claims. It then covers the collateral types where filing does nothing, the buyer who takes free of a perfected interest, the federal regimes that displace the state system entirely, and what happens to a floating lien on the day a bankruptcy petition is filed. It closes with enforcement, where commercial reasonableness governs every aspect of a disposition.