Courts sometimes confront gaps in formal law where doctrinal sources like text, history, and precedent fail to offer guidance in resolving a particular case. When these gaps are narrow, judges can generally address them through analogical reasoning or intuition. But sometimes legal gaps are too substantial to be filled with one-off decisions, and judges are called upon to create whole legal tests without the benefit of formal guidance or constraint. Courts currently lack a theoretical framework for addressing these difficult situations. This Article analyzes these “legal blank slates” and provides a framework for addressing them. Blank slates are less common than other types of legal indeterminacy, like interpretive controversies, institutional conflicts, and narrow formal gaps. But they arise fairly regularly and often involve important legal issues. This Article surveys examples of legal blank slates in areas like Fourth Amendment law, free speech, the dormant Commerce Clause, and anti-discrimination law, and draws lessons for a general theory of blank slates. This Article also offers several strategies that courts might use to effectively address blank slates and develops a framework for choosing the best approach for a given situation. Ultimately, blank slate theory can shed light on concrete doctrinal questions as well as broader debates about legal interpretation. It can, for example, suggest a new approach for determining the Fourth Amendment’s scope and help explain why previous Fourth Amendment regimes have been unsuccessful. More generally, the theory can provide a unique perspective on interpretive debates, using the extreme case of blank slates to gain fresh insights into legal interpretation as a whole.