The precautionary principle is accepted in India as a fundamental tool to promote sustainable development and is employed within Indian environmental governance to promote better health and environmental decisions. Scientific uncertainty is at the core of the precautionary principle. The application of the precautionary principle is an open-ended issue. This article seeks to add to the limited empirical studies on the understanding, appreciation and application of the precautionary principle by key environmental actors, as differing legal responses and decisions may be irreversible before conclusive scientific knowledge and evidence become available. Building on researcher’s unique Indian data, and drawing on the theoretical insights developed by Charles Weiss, an explanatory environmental framework addresses the uncertainty of science by assembling a scale of legal standards arranged in a hierarchy of levels of increasing certainty familiar to lawyers and the judiciary. Reported Indian cases from the Supreme Court and the National Green Tribunal are selected to illustrate levels of scientific certainty or uncertainty and corresponding legal standards of proof constituting acceptable bases for legal decisions in practical context especially the precautionary principle. The article suggests India should develop a framework of guidelines that would provide an effective roadmap for decision-makers applying the precautionary principle.