Abstract In this paper, we develop a neural summarization model which can effectively process multiple input documents and distill abstractive summaries. Our model augments a previously proposed Transformer architecture (Liu et al., 2018) with the ability to encode documents in a hierarchical manner. We represent cross-document relationships via an attention mechanism which allows to share information as opposed to simply concatenating text spans and processing them as a flat sequence. Our model learns latent dependencies among textual units, but can also take advantage of explicit graph representations focusing on similarity or discourse relations. Empirical results on the WikiSum dataset demonstrate that the proposed architecture brings substantial improvements over several strong baselines