Transfer learning from natural image datasets, particularly I N , using standard large models and corresponding pretrained weights has become a de-facto method for deep learning applications to medical imaging. However, there are fundamental di erences in data sizes, features and task speci cations between natural image classi cation and the target medical tasks, and there is little understanding of the e ects of transfer. In this paper, we explore properties of transfer learning for medical imaging. A performance evaluation on two large scale medical imaging tasks shows that surprisingly, transfer o ers little bene t to performance, and simple, lightweight models can perform comparably to I N architectures. Investigating the learned representations and features, we nd that some of the di erences from transfer learning are due to the over-parametrization of standard models rather than sophisticated feature reuse. We isolate where useful feature reuse occurs, and outline the implications for more e cient model exploration. We also explore feature independent bene ts of transfer arising from weight scalings.