Grammatical Error Correction (GEC) has been recently modeled using the sequenceto-sequence framework. However, unlike sequence transduction problems such as machine translation, GEC suffers from the lack of plentiful parallel data. We describe two approaches for generating large parallel datasets for GEC using publicly available Wikipedia data.The first method extracts sourcetarget pairs from Wikipedia edit histories with minimal filtration heuristics, while the second method introduces noise into Wikipedia sentences via round-trip translation through bridge languages. Both strategies yield similar sized parallel corpora containing around 4B tokens. We employ an iterative decoding strategy that is tailored to the loosely supervised nature of our constructed corpora. We demonstrate that neural GEC models trained using either type of corpora give similar performance. Fine-tuning these models on the Lang-8 corpus and ensembling allows us to surpass the state of the art on both the CoNLL-2014 benchmark and the JFLEG task. We provide systematic analysis that compares the two approaches to data generation and highlights the effectiveness of ensembling. * * Equal contribution. Listing order is random. Jared conducted systematic experiments to determine useful variants of the Wikipedia revisions corpus, pre-training and finetuning strategies, and iterative decoding. Chris implemented the ensemble and provided background knowledge and resources related to GEC. Shankar ran training and decoding experiments using round-trip translated data. Jared, Chris and Shankar wrote the paper. Noam identified Wikipedia revisions as a source of training data. Noam developed the heuristics for using the full Wikipedia revisions at scale and conducted initial experiments to train Transformer models for GEC. Noam and Niki provided guidance on training Transformer models using the Tensor2Tensor toolkit. Simon proposed using round-trip translations as a source for training data, and corrupting them with common errors extracted from Wikipedia revisions. Simon generated such data for this paper.