Title: Contextual HyperNetworks  for Novel Feature Adaptation. Abstract: While deep learning has obtained state-of-the-art results in many applications, the adaptation of neural network architectures to incorporate new features remains a research challenge. This issue is particularly severe in online learning settings, where new features are added continually with few or no associated observations. As such, methods for adapting neural networks to novel features which are both time and data-efficient are desired. To address this, we propose the Contextual HyperNetwork (CHN), which predicts the network weights associated with new features by incorporating information from both existing data as well as the few observations for the new feature and any associated feature metadata. At prediction time, the CHN requires only a single forward pass through a small neural network, yielding a significant speed-up when compared to re-training and fine-tuning approaches. In order to showcase the performance of CHNs, in this work we use a CHN to augment a partial variational autoencoder (P-VAE), a flexible deep generative model which can impute the values of missing features in sparsely-observed data. We show that this system obtains significantly improved performance for novel feature adaptation over existing imputation and meta-learning baselines across recommender systems, e-learning, and healthcare tasks.