Title: On Nondeterminism and Instability in Neural Network Optimization. Abstract: Optimization nondeterminism causes uncertainty when improving neural networks, with small changes in performance difficult to discern from run-to-run variability. While uncertainty can be reduced by training multiple copies of a model with different random seeds, doing so is time-consuming, costly, and makes reproducibility challenging. Despite this, little attention has been paid towards establishing an understanding of this problem. In this work, we establish an experimental protocol for understanding the effect of optimization nondeterminism on model diversity, which allows us to study the independent effects of a variety of sources of nondeterminism. Surprisingly, we find that each source of nondeterminism all have similar effects on multiple measures of model diversity. To explain this intriguing fact, we examine and identify the instability of model training, when taken as an end-to-end procedure, as the key determinant. We show that even one-bit changes in initial model parameters result in models that converge to vastly different values. Last, we demonstrate that recent methods in accelerated model ensembling hold promise for reducing the effects of instability on run-to-run variability.