Title: Manifold Mixup: Learning Better Representations by Interpolating Hidden States. Abstract: Deep networks often perform well on the data distribution on which they are trained, yet give incorrect (and often very confident) answers when evaluated on points from off of the training distribution. This is exemplified by the adversarial examples phenomenon but can also be seen in terms of model generalization and domain shift.  Ideally, a model would assign lower confidence to points unlike those from the training distribution.  We propose a regularizer which addresses this issue by training with interpolated hidden states and encouraging the classifier to be less confident at these points.  Because the hidden states are learned, this has an important effect of encouraging the hidden states for a class to be concentrated in such a way so that interpolations within the same class or between two different classes do not intersect with the real data points from other classes.  This has a major advantage in that it avoids the underfitting which can result from interpolating in the input space.  We prove that the exact condition for this problem of underfitting to be avoided by Manifold Mixup is that the dimensionality of the hidden states exceeds the number of classes, which is often the case in practice.  Additionally, this concentration can be seen as making the features in earlier layers more discriminative.  We show that despite requiring no significant additional computation, Manifold Mixup achieves large improvements over strong baselines in supervised learning, robustness to single-step adversarial attacks, semi-supervised learning, and Negative Log-Likelihood on held out samples.