Write first.
Cite later.

Run by the U.S. National Library of Medicine and considered the gold standard for biomedical research, indexing 35M+ peer-reviewed citations. Best for medicine, biology, nursing, public health, neuroscience, and psychology papers.

A Cornell-hosted preprint server where papers are moderated but not peer-reviewed, so results are cutting-edge but unsettled. Best for CS, machine learning, physics, math, and statistics, fields where the freshest work lives here before formal publication.

A trusted cross-field index by the Allen Institute for AI covering ~200M papers; the index is reputable, but any individual paper's credibility still depends on its venue. Best for general academic search, finding highly-cited work, and tracing citation graphs.

An open, fully-curated catalog of the global research system (successor to Microsoft Academic Graph) with strong metadata and broad non-English coverage. Best for humanities and social-science searches and finding open-access versions outside the major Anglo-American journals.

The official DOI registry for scholarly publishers, making it authoritative for publication metadata. If a paper has a DOI, CrossRef knows about it. Best for humanities, history, literature, religion, and law journal articles and for verifying exact citation details.