Tort law responds to discrete, harmful events—“accidents”—by converting unruly facts into a binary on/off judgment about liability. This operation, characteristic of much of law, resembles the “thresholding” process used to convert grayscale images to black and white. It embeds decisions about how to isolate and evaluate the sample of risk-related behavior connected to the accident. This Article focuses on the implicit but powerful role that aggregation—of behavior, precautions, and events—plays in the determination of liability. These aggregative choices determine how large a slice of an injurer’s conduct tort law will capture within its viewfinder, and how tight the causal connection must be between the shortfalls observed there and the accident at hand. The analysis here also sheds light on questions of legal thresholding that emerge in other doctrinal areas.