Monitoring or supervision in the sense of observing behaviour to establish whether the observed party has acted according to the applicable norms or stan-dards of behavior is one of the many possibilities to influence behaviour. In this paper, several legal instruments (tort law, regulation and criminal law) are ana-lysed as instruments to induce parties to behave according to the rules. Those instruments are divided on the basis of when the norm is formulated (before or after the externality is caused), who initiates enforcement of the norm (individ-ual people or the government), how the instrument works (directly or indirectly) and the stage of legal intervention (preclusion, act-based sanctions and harm-based sanctions). It is argued that all instruments have strong and weak points, and that a combination of instruments is necessary. The costs of monitoring and supervision are relevant in determining the optimal mix of enforcement instru-ments, but are not all-decisive.