Title: The Geometry of Adversarial Subspaces. Abstract: Artificial neural networks (ANNs) are constructed using well-understood mathematical operations, and yet their high-dimensional, non-linear, and compositional nature has hindered our ability to provide an intuitive description of how and why they produce any particular output. A striking example of this lack of understanding is our inability to design networks that are robust to adversarial input perturbations, which are often imperceptible to a human observer but cause significant undesirable changes in the network’s response. The primary contribution of this work is to further our understanding of the decision boundary geometry of ANN classifiers by utilizing such adversarial perturbations. For this purpose, we define adversarial subspaces, which are spanned by orthogonal directions of minimal perturbation to the decision boundary from any given input sample. We find that the decision boundary lies close to input samples in a large subspace, where the distance to the boundary grows smoothly and sub-linearly as one increases the dimensionality of the subspace. We undertake analysis to characterize the geometry of the boundary, which is more curved within the adversarial subspace than within a random subspace of equal dimensionality. To date, the most widely used defense against test-time adversarial attacks is adversarial training, where one incorporates adversarial attacks into the training procedure. Using our analysis, we provide new insight into the consequences of adversarial training by quantifying the increase in boundary distance within adversarial subspaces, the redistribution of proximal class labels, and the decrease in boundary curvature.