This guide covers consensual security interests and non-consensual liens from documentation through enforcement. It sets out attachment, the methods of perfection and which collateral each reaches, and the priority rules that decide competing claims, including the purchase money exception and the buyer in ordinary course rule. It covers default, self-help repossession and the breach of the peace limit, judicial process, notice requirements, commercially reasonable disposition, deficiency and surplus, and redemption. It then treats mechanics liens, tax liens, judgment liens, and statutory liens as the separate systems they are, with their notice and recording deadlines. Bankruptcy occupies its own part: the automatic stay, valuation, avoidance powers, and the survival of liens through discharge. Diligence, documentation, and portfolio maintenance close it out.