Editor's Summary: The federal courts of appeal are currently engaged in debate over the increase in probability of future harm that must be demonstrated by petitioners to establish a cognizable injury for Article III standing purposes. Two recent decisions in the U.S. Court of Appeals for the District of Columbia (D.C.) Circuit--one withdrawn and replaced by the other--may herald a new, quantitative approach to standing analysis with great implications for environmental law. In this Article, Cassandra Sturkie and Nathan Seltzer review these D.C. Circuit developments and what they mean for cases in which purely "probabilistic" environmental or health injuries are alleged. They examine the constitutional requirement of injury-in-fact and the D.C. Circuit's unique precedent in cases involving increased risk of future harm. They discuss the striking differences in the court's position and tone in the NRDC I and NRDC II cases. Finally, they conclude with lessons for environmental law practitioners and their clients.