In federal courts sitting in diversity, which doctrine governs whether state substantive law or federal procedural law applies?

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In federal courts sitting in diversity, the governing rule is the Erie doctrine. It requires applying state substantive law to determine rights and obligations, while using federal procedural rules to manage the litigation. This separation avoids federal courts crafting their own substantive rules and keeps the outcome of the case aligned with what would happen if it were brought in state court. The Supremacy Clause states that federal law is the supreme law, but it isn’t the framework for deciding which state versus federal law governs a particular issue in diversity cases—that role is Erie. The Rules Enabling Act governs what federal procedural rules can do, but it doesn’t determine substantive versus procedural choice in this setting. Preemption concepts address conflicts between federal and state law more broadly, not the core Erie allocation in diversity.