The success of any groundwater management plan depends on user compliance. There is an intimate relationship between regulatory regimes and pumper perceptions. As well as its enforcement powers, an agency's behavior sends information to users. While enforcement power need not always be used to be effective, it must be seen as credible as well as legitimate. Perceived legitimacy has different sources – or may be lacking – depending on the origins, and implementation, of the regulatory apparatus. This paper examines a number of California groundwater basins, employing variables from Ostrom's analytical frameworks. In comparison with a West Australian regulated basin - where compliance is low, monitoring weak, and enforcement ineffective - we examine the effect on compliance of the adjudicated basin approach. We focus on the role of enforcement provisions, and their origins and implementation, in shaping appropriator attitudes towards compliance. Key attributes of effective systems include perceived legitimacy among users, mutual visibility of actions, and the credible threat of enforcement or sanction. We examine the extent to which 'administrative adjudications' may more cost-effectively provide the benefits of court adjudications. The paper illustrates that monitoring and enforcement are more effective and less costly when institutions encourage cooperation than when they promote competition. While norms, social capital, and trust must bear upon and inform the types of rules chosen at the collective-choice level, they also arise from the operation of those rules – i.e., from users' iterative reactions to the arrangements chosen. Groundwater management plans should incorporate design elements encouraging collaborative attitudes among users.