This dissertation investigates the optimality of precautions taken by victims, gatekeepers, and online copyright enforcers. The overarching theme is to identify the inefficiencies of private precaution-taking behavior, along with their underlying causes, and create legal regimes to address such inefficiencies. On the one hand, precautions can help deter sociallyinefficient misconduct, which is already regulated by law enforcement. On the other hand, individuals may not fully internalize the costs associated with their precautions. Therefore, private precautionary decisions can be both socially suboptimal and sensitive to the strategies or focuses of law enforcement. Designing appropriate legal regimes should consider the social benefit of deterrence and the total social cost borne by precaution takers, wrongdoers, and non-perpetrator parties. This dissertation contains three main chapters. Chapter 2 deals with the most essential case: victims, investigating how to regulate their precautionary measures when inflicting harm on both wrongdoers and non-perpetrator third parties. Chapter 3 extends the analysis and applies the insights from the victim-precaution literature to the context of gatekeeper liability, identifying the externalities of gatekeeping and its sensitivity to law enforcement strategies, and providing guidance on designing appropriate gatekeeper liability regimes. Chapter 4 shifts to a specific context of online copyright enforcement and proposes a regime to hold copyright holders accountable for the over-removal problem.